Section 6: Freeways
Anchor: #i1084030Overview
A freeway is defined as a controlled access multilane divided facility. Freeways are functionally classified as arterials but have unique design characteristics that set them apart from non-access controlled arterials. This section discusses the features and design criteria for freeways and includes the following subsections:
- Basic Design Criteria
- Access Control
- Mainlane Access
- Vertical and Horizontal Clearance at Structures
- Frontage Roads
- Interchanges
Basic Design Criteria
Specific references to Freeway Geometric Design criteria are shown in Table 3-15:
|
Design Criteria |
Reference |
|---|---|
|
General |
|
|
Horizontal Clearance |
|
|
Grades |
|
|
Minimum Horizontal Radius |
|
|
Superelevation |
Tables 2-6, 2-7 and 2-8 |
|
Vertical Curvature |
|
|
Pavement Cross Slope |
Chapter 2, Pavement Cross Slope |
|
Freeways |
|
|
Design Speed Mainlanes (urban and rural) |
|
|
Design Speed Frontage Roads (urban and rural) |
Chapter 3, Frontage Roads |
|
Capacity and LOS Analysis |
Highway Capacity Manual |
Anchor: #CHDEHIJC
Access Control
This subsection discusses access control and includes the following topics:
Anchor: #CHDHCHCJGeneral
The entire Interstate Highway System and portions of the State Highway System have been designated by the Commission as Controlled Access Highways, thereby making it necessary along certain sections of said highways to either limit or completely deny the abutting owner’s access rights, which include the right of ingress and egress and the right of direct access to and from said owner’s abutting property to said highway facility. Such access may be controlled under the State’s Police Power, which is an inherent right of a sovereignty. However, the existing right of access to an existing public way is an increment of ownership and a part of the bundle of rights vested in the owner of abutting property. It is a legal right, and though such right may be limited or completely denied under the State’s Police Power, the owner is entitled to be paid whatever damages may be suffered by reason of the loss of such access.
The abutting owners are denied access to any controlled access highway on new location, unless there is a specific grant of access, and no damages may be claimed for the denial of access to the new facility; the theory being that the owner cannot be damaged by the loss of something which the owner never had.
If an existing road is converted into a controlled access facility, the design of which does not contemplate the initial construction of frontage roads, and the abutting owner is to be denied access to such facility pending frontage road construction, there is a taking of the owner’s access rights. If an existing road is converted into a controlled access facility, the design of which does contemplate frontage road(s) in the initial construction, and the abutting owner is not to be denied access to such frontage road(s), there is not taking or denial of access rights. Access to the frontage road(s) constitutes access to the facility. Further control of movements, once upon the frontage road, such as one-way traffic, no U-turns, no left or right turns, denial of direct access to the through lanes, and circuitous routes are all controlled under police power and inflict no more control over the abutting owner than is inflicted upon the general public.
If an existing road is converted into a controlled access facility and no part of the abutting owner’s property is taken for right of way, but access is to be denied to the controlled access facility, and by reason of such denial of access it is found that such owner will suffer damages measured by the diminution of the market value of said abutting land, said owner should be requested to release and relinquish said access rights for consideration equal to the State’s approved value for such damages. If the owner is not willing to negotiate on these terms, then the access right may be acquired through eminent domain proceedings. In some instances, the State’s appraisal and approved value may indicate that there is no diminution in value by reason of the access denial, and in those cases the abutting owner should be requested to release and relinquish access rights for no cash consideration. If the owner refuses to do so, then the access rights should be acquired through eminent domain proceedings with the State testifying to a zero value for such rights.
Anchor: #CHDGJDJCMainlane Access
Freeway mainlane access, either to or from abutting property or cross streets, is only allowed to occur through a ramp. This control of mainlane access may be achieved through one of the following methods:
- through access restrictions whereby the access to the highway from abutting property owners is denied with ingress and egress to the mainlanes only at selected freeway or interchange ramps
- through construction of frontage roads permitting access
to the mainlanes only at selected ramps.
In either case, direct access from private property to the mainlanes is prohibited without exception.
Frontage Road Access
In the case where frontage roads are provided, access should be controlled for operational purposes at ramp junctions with frontage roads through access restrictions or the use of the State’s police powers to control driveway location and design. Figures 3-13 and 3-14 show recommended access control strategies for planned exit and entrance ramps, respectively, and should be used where practical.
Figure 3-13. Recommended Access Control At Exit Ramp Junction With Frontage Road. Click here to see a PDF of the image.
Figure 3-14. Recommended Access Control At Entrance Ramp Junction With Frontage Road. Click here to see a PDF of the image.
Anchor: #CHDBFFDCDriveways and Side Streets
The placement of streets and driveways in the vicinity of freeway ramp/frontage road intersections should be carefully considered and permitted only after local traffic operations are considered. Information on the driveway clearance from the cross street intersection is contained in the TxDOT Access Management Manual and should be considered in the locating of any driveways on projects involving the construction or reconstruction of ramps and/or frontage roads.
Table 3-16 shows the spacing to be used between exit ramps and driveways, side streets, or cross streets if practical. The number of weaving lanes is defined as the total number of lanes on the frontage road downstream from the ramp.
|
Total Volume (Frtg rd +Ramp) (vph) |
Driveway or Side Street Volume (vph) |
Spacing (ft [m]) |
||
|---|---|---|---|---|
|
-- |
-- |
Number of Weaving Lanes |
||
|
-- |
-- |
2 |
3 |
4 |
|
< 2500 |
< 250 |
460 [140] |
460 [140] |
560 [170] |
|
-- |
> 250 |
520 [160] |
460 [140] |
560 [170] |
|
-- |
> 750 |
790 [240] |
460 [140] |
560 [170] |
|
-- |
> 1000 |
1000 [300] |
460 [140] |
560 [170] |
|
> 2500 |
< 250 |
920 [280] |
460 [140] |
560 [170] |
|
-- |
> 250 |
950 [290] |
460 [140] |
560 [170] |
|
-- |
> 750 |
1000 [300] |
600 [180] |
690 [210] |
|
-- |
> 1000 |
1000 [300] |
1000 [300] |
1000 [300] |
Driveway or side street access on the frontage road in close downstream proximity to exit ramp terminals increases the weaving that occurs on the frontage road and may lead to operational problems. For this reason, it is important to maintain appropriate separation between the intersection of the exit ramp and frontage road travel lanes, and downstream driveways or side streets where practical.
It is recognized that there are occasions when meeting these exit ramp separation distance values may not be possible due to the nature of the existing development, such as a high number of closely spaced driveways and/or side streets especially when in combination with closely spaced interchanges. In these cases, at least 250 ft [75 m] of separation should be provided between the intersection of the exit ramp and frontage road travel lanes and the downstream driveway or side street. Since the use of only 250 ft [75 m] of separation distance may negatively impact the operation of the frontage road, exit ramp, driveway and/or side street traffic, careful consideration should be given to its use. When the 250 ft [75 m] separation distance cannot be obtained, consideration should be given to channelization methods that would restrict access to driveways within this 250 ft [75 m] distance. Refer to the Texas MUTCD for specific types of channelization.
There will be similar occasions when meeting the entrance ramp separation distance values may not be possible due to the same existing development conditions associated with exit ramps. In these cases, at least 100 ft [30 m] of separation distance should be provided between the intersection of the entrance ramp and frontage road travel lanes and the upstream driveway or side street. Since the use of only 100 ft [30 m] of entrance ramp separation distance may also negatively impact the operation of the frontage road, entrance ramp, driveway, and/or side street traffic, careful consideration should be given to its use. As with exit ramps, when the 100 ft [30 m] entrance ramp separation distance cannot be obtained, consideration should be given to channelization methods that would restrict access to driveways within this 100 ft [30 m] distance. Refer to the Texas MUTCD for specific types of channelization.
Relocating driveways. On reconstruction projects, it may be necessary to close or relocate driveways in order to meet these guidelines. However, if the closure/relocation is not feasible, and adjustment of the location of the ramp gore along the frontage road is not practical, then deviation from these recommended guidelines may be necessary.
Ramp Location. In the preparation of schematic drawings, care should be exercised to develop design in sufficient detail to accurately tie down the locations of ramp junctions with frontage roads and thus the location of access control limits. These drawings are often displayed at meetings and hearings and further become the basis for right-of-way instruments or, in some cases, the Department's regulation of driveway location.
In some instances, ramps must be shifted to satisfy level of service considerations or geometric design controls. When this is necessary, the access control limits should also be shifted if right-of-way has not been previously purchased.
Anchor: #CHDBHJFBMethods
A controlled access highway may be developed in either of two ways:
- Designation (Transportation Code §203.031 and access restrictions)
- Design (continuous frontage road and State’s police power)
Designation
When the Texas Transportation Commission designates a freeway to be developed as a controlled access facility under Transportation Code §203.031, the State is empowered to control access through access restrictions. All Interstate Highways are designated as controlled access and certain other routes have been or may be designated. These designated freeways may or may not have frontage roads, whichever arrangement is determined to be appropriate as discussed in Planning Development of freeways by designation, rather than solely by design, is the preferred design approach especially for all new location freeways.
Under Transportation Code §203.031, Not Along An Existing Public Road. Whenever designated controlled access freeways include frontage roads and the planned location is not along an existing public road, preferably access should be controlled through access restrictions at ramp junctions with frontage roads as shown on Figure 3-13 and Figure 3-14.
Where no frontage roads are provided, access is controlled to the mainlanes by access restriction.
Under Transportation Code §203.031, Along An Existing Public Road. Whenever a designated controlled access freeway is to be provided along the location of an existing public road, generally (subject to discussion in Planning) frontage roads are provided to retain or restore existing access.
Frontage road access should be controlled by imposing access restrictions in accordance with Figure 3-13 and Figure 3-14 whenever all of the following conditions prevail:
- Right-of-way is being obtained from the abutting property owner(s).
- A landlocked condition does not result.
- Recommended control of access as shown in
Figure
3-13 and
Figure 3-14.
Access may be controlled by use of the State's police power to control driveway location and design where any of the following conditions prevail:
- No right of way is obtained from the abutting property owner(s).
- Restricting access results in landlocking an abutting property.
Whenever the State's police powers are used, the denial of access zone should be free of driveways insofar as practical.
Anchor: #CHDCIEGBDesign
If an existing highway is to be developed as a controlled access facility solely by design (not designated by the Transportation Commission), the Texas Department of Transportation is not empowered to purchase access rights but must achieve access control by construction of continuous frontage roads and by the utilization of the State's police power to control driveways, particularly at locations such as ramp junctions with frontage roads.
In the interest of providing for highway safety and utility, the State may regulate driveway location and design through its police powers. Landlocking through complete denial of access is beyond the State's regulatory power (without Commission designation under the Transportation Code). The State, however, may effectively regulate driveway location in accordance with Statewide policy as long as the following two conditions are met:
- Reasonable access is provided.
- Landlocking of an abutting property does not result.
The Departmental publication entitled TxDOT Access Management Manual governs design and location of driveways.
Whenever new or relocated ramps are to be provided along existing freeways, the design philosophy shown in Frontage Roads applies. Access should therefore be controlled at frontage road junctions through access restriction as illustrated in Figures 3-13 and 3-14 if practical and feasible.
Whenever access is to be controlled solely by provision of frontage roads, departmental power to regulate driveway location and design should be used to control access near ramp junctions. However, where designation by the Transportation Commission is practical, it is preferred over controlling access solely by design.
Anchor: #i1084372Mainlanes
This subsection discusses mainlanes and includes information on the following topics:
- Design Speed
- Level of Service
- Lane Width and Number
- Shoulders
- Medians
- Outer Separation
- Crossing Facilities
Design Speed
The design speed of urban freeways should reflect the desired operating conditions during non-peak hours. The design speed should not exceed the limits of prudent construction, right-of-way, and socioeconomic costs because a large proportion of vehicles are accommodated during periods of peak flows when lower speeds are tolerable. Design speeds for rural freeways should be high, providing a design speed that is consistent with the overall quality and safety of the facility.
Table 3-17 provides minimum design speeds for freeways:
|
Facility |
Minimum |
|---|---|
|
Mainlanes - Urban |
50 [80] |
|
Mainlanes - Rural |
70 [110] |
Anchor: #CHDHHIGB
Level of Service
For acceptable degrees of congestion, urban freeways and their auxiliary facilities should generally be designed for level of service C, as defined in the Highway Capacity Manual, in the design year. In heavily developed urban areas, level of service D may be acceptable. In rural areas, level of service B is desirable for freeway facilities; however, level of service C may be acceptable for auxiliary facilities (i.e., ramps, direct connections and frontage roads) carrying unusually high volumes.
Anchor: #CHDDIICJLane Width and Number
The minimum and usual mainlane width is 12 ft [3.6 m]. The number of lanes required to accommodate the anticipated traffic in the design year is determined by the level of service evaluation as discussed in the Highway Capacity Manual. See Table 3-18: Roadway Widths for Controlled Access Facilities and Figure 3-11 for further information.
Figure 3-15. (US). Typical Freeway Sections. Click US Customary or Metric to see a PDF of the image.
Anchor: #CHDEJFHGShoulders
Continuous surfaced shoulders are provided on each side of the mainlane roadways, both rural and urban, as shown in Figure 3-15. The minimum widths should be 10 ft [3.0 m] on the outside and 4 ft [1.2 m] on the median side of the pavement for four-lane freeways. On freeways of six lanes or more, 10 ft [3.0 m] inside shoulders for emergency parking should be provided. A 10 ft [3.0 m] outside shoulder should be maintained along all speed change lanes with a 6 ft [1.8 m] shoulder considered in those instances where light weaving movements take place. See Table 3-18: Roadway Widths for Controlled Access Facilities and Figure 3-11 for further information.
Anchor: #CHDDGDCEMedians
The width of the median is the distance between the inside edges of the travel lanes. For depressed freeway sections, medians 76 ft [22.8 m] in width are generally used. Where topography, right-of-way, or other special considerations dictate, depressed freeway median width may be reduced from 76 ft [22.8 m] to a minimum of 48 ft [14.4 m]. A median width of 24 ft to 30 ft [7.2 m to 9.0 m] is generally used on freeway sections with flush medians. On freeways including six or more travel lanes and a flush 24 ft [7.2 m] median, the resulting section provides for 10 ft [3.0 m] inside shoulders and a usual 2 ft [0.6 m] offset to barrier centerline. See Figure 3-11 for further information.
Because of high speed and volume traffic on urban freeways and the resulting adverse environment for accomplishing construction improvements thereon, it is the usual practice to construct the ultimate freeway section initially. Under those unusual circumstances where future additional lanes will be provided in the median area, the usual median width of 24 ft [7.2 m] should be increased by the appropriate multiple of 12 ft [3.6 m] in anticipation of need for additional lanes. Provisions should be made, or retained, for any future high occupancy vehicle lanes in the median.
At horizontal curves on freeways with narrow medians, a check should be made to insure that the median barrier does not restrict stopping sight distance to less than minimum values.
For information on freeway median crossings, refer to Chapter 7, Emergency Median Openings on Freeways.
Anchor: #CHDIICJIOuter Separation
The portion of the freeway between the mainlanes and frontage road, or right-of-way line where frontage roads are not provided, should be wide enough to accommodate shoulders, speed change lanes, side slopes and drainage, retaining walls and ramps, as well as the necessary signs and other appurtenances necessary for traffic control. Because of right-of-way limitations in urban areas, the outer separation may oftentimes be narrower than desired; however, in rural areas, where opposing headlights along a two-way frontage road tend to reduce a driver’s comfort and perception on the freeway, the outer separation should be as wide as possible.
Anchor: #CHDHIEJECrossing Facilities
The following exhibits show the appropriate widths for facilities crossing the freeway:
- Urban Streets: Table 3-1: Geometric Design Criteria for Urban Streets
- Suburban Roadways: Table 3-5: Geometric Design Criteria for Suburban Roadways
- Rural Two-Lane Highways: Table 3-7. Geometric Design Criteria for Rural Two-Lane Highways and Table 3-8: Width of Travel Lanes and Shoulders on Rural Two-lane Highways
- Multilane Rural Highways : Table 3-12: Design Criteria For Multilane Rural Highways (Non-controlled Access) (All Functional Classes)
The Bridge Project Development Manual should also be referenced for appropriate structure widths.
Anchor: #CHDGBIHCVertical and Horizontal Clearance at Structures
Vertical. All controlled access highway grade separation structures, including railroad underpasses, should provide 16.5 ft [5.0 m] minimum vertical clearance over the usable roadway.
Structures over the mainlanes of interstate or controlled access highways must meet the minimum vertical clearance requirement except within cities where the 16.5 ft [5.0 m] vertical clearance is provided on an interstate loop around the particular city. Less than 16 ft [4.9 m] vertical clearance on rural interstate and single priority defense interstate routes, including ramps and collector-distributor roads, requires approval through the Design Division with the Federal Highway Administration and/or the Military Traffic Management Command Transportation Engineering Agency (MTMCTEA) of the Department of Defense (DOD).
Roadways under the mainlanes of interstate or controlled access highways must meet the minimum vertical clearance requirements for the appropriate undercrossing roadway classification.
Vertical clearances for pedestrian crossover structures should be approximately 1 ft [0.3 m] greater than that provided for other grade separation structures. This is due to the increased risk of personal injury upon impact by over-height loads and the relative weakness of such structures to resist lateral loads from vehicular impact.
The above-specified clearances apply over the entire width of roadway including usable shoulders and include an allowance of 6 inches [150 mm] for future pavement overlays. It is recognized that it is impractical to arrive at the exact clearance dimensions on the structure plans. However, the above clearances should not generally be exceeded by more than approximately 3 inches [75 mm].
Vertical clearance for railroad overpasses is shown in Figure 3-16, and is discussed further in the Bridge Project Development Manual.
Figure 3-16. Typical Highway Railway Railroad Overpass. Click here to see a PDF of the image.
Horizontal. The minimum horizontal clearance to bridge parapets and piers should be as shown in Table 2-11: Horizontal Clearances and Figure 3-12.
|
(US Customary) |
||||
|---|---|---|---|---|
|
Type of Roadway |
Inside Shoulder Width (ft) |
Outside Shoulder Width2 (ft) |
Traffic Lanes (ft) |
|
|
Mainlanes: |
- |
- |
- |
|
|
4-Lane Divided |
4 |
10 |
24 |
|
|
6-Lane or more Divided |
10 |
10 |
361 |
|
|
1-Lane Direct Conn.2 |
2 Rdwy.; 4 Str. |
8 |
14 |
|
|
2-Lane Direct Conn. |
2 Rdwy.; 4 Str. |
8 |
24 |
|
|
Ramps2 (uncurbed) |
2 Rdwy.; 4 Str. |
Min. |
Des. |
14 |
|
- |
- |
6 |
8 |
- |
|
Ramps3 (curbed) |
— |
— |
22 |
|
|
(Metric) |
||||
|
Type of Roadway |
Inside Shoulder Width (m) |
Outside Shoulder Width2 (m) |
Traffic Lanes (m) |
|
|
Mainlanes: |
- |
- |
- |
|
|
4-Lane Divided |
1.2 |
3.0 |
7.2 |
|
|
6-Lane or more Divided |
3.0 |
3.0 |
10.81 |
|
|
1-Lane Direct Conn.2 |
0.6 Rdwy.; 1.2 Str. |
2.4 |
4.2 |
|
|
2-Lane Direct Conn. |
0.6 Rdwy.; 1.2 Str. |
2.4 |
7.2 |
|
|
Ramps2 (uncurbed) |
0.6 Rdwy.; 1.2 Str. |
Min. |
Des. |
4.2 |
|
- |
- |
1.8 |
2.4 |
- |
|
Ramps3 (curbed) |
— |
— |
6.6 |
|
|
1 For more than six lanes, add 12 ft [3.6 m] width per lane. 2 If sight distance restrictions are present due to horizontal curvature, the shoulder width on the inside of the curve may be increased to 8 ft [2.4 m] and the shoulder width on the outside of the curve decreased to 2 ft [0.6 m] (Rdwy) or 4 ft [1.2 m] (Str). 3 The curb for a ramp lane will be mountable and limited to 4 inches [100 mm] or less in height. The width of the curbed ramp lane is measured face to face of curb. Existing curb ramp lane widths of 19 ft [5.7 m] may be retained. |
||||
Anchor: #CHDJCDDD
Frontage Roads
This subsection discusses frontage roads and includes information on the following topics:
Anchor: #CHDHAGIFFunction and Uses
Frontage roads serve a multitude of purposes in addition to controlling or providing access. Urban frontage roads are multi-functional. They reduce the “barrier” effect of urban freeways since they provide for some of the circulation of the local street system. They provide invaluable operational flexibility, serving as detour routes when mainlane accidents occur, during mainlane maintenance activity, for over-height loads, as bus routes, or during inclement weather. For freeways that include freeway surveillance and control, continuous frontage roads provide the operational flexibility required to manage saturation.
In addition to the above-described purposes of frontage roads, many times they prove advantageous when used as the first stage of construction for an ultimate freeway facility. By constructing frontage roads prior to the mainlanes, interim traffic demands very often can be satisfied and a usable section of highway can be opened to the traveling public at a greatly reduced cost.
Anchor: #CHDFIGBAPlanning
Frontage roads may be incorporated into a project at various points during the project development, however, later incorporation of frontage roads will be more difficult. Frontage roads may be included:
- during the planning stage
- subsequent to the planning stage
- after the freeway has been constructed.
Frontage road construction may be funded by TxDOT, a local government, or shared by both. The Texas Transportation Commission has adopted rules governing the construction and funding of frontage roads. All frontage road development must be in accordance with the rules contained in 43 Texas Administrative Code (TAC) §15.54. The Project Development Policy Manual can also be referenced for additional information.
Changes in control of access must be in accordance with 43 TAC §15.54(d)(4).
As specified in the Right of Way Manual, Volume 1, subsequent changes in the control of access will be as shown on approved construction plans or as provided in instruments conveying right-of-way on authorized projects, or as may be authorized by Commission Minute Order. Where access is permitted to adjacent properties, ingress and egress will be governed by the issuance of permits to construct access driveway facilities as set forth in established Departmental policy which is designed to provide reasonable access, to insure traffic safety, and preserve the utility of highways.
Anchor: #CHDFIAGHDesign Speed on Frontage Roads
Design speeds for frontage roads are a factor in the design of the roadway. For consistency, design speeds should be used that match values used for collector streets or highways. For urban frontage roads, the desirable design speed is 50 mph [80 km/h] and the minimum design speed is 30 mph [50 km/h]. See Table 3-5: Geometric Design Criteria for Suburban Roadways for design speeds for suburban frontage roads, and Table 3-6: Minimum Design Speed for Rural Two-lane Highways for rural frontage roads.
Anchor: #CHDFEBJACapacity and Level of Service
Although techniques to estimate capacity and level of service on freeways and urban arterials are detailed in the Highway Capacity Manual, these procedures should not be applied directly to frontage roads, as frontage roads have features characteristic of both freeways (i.e., exit and entrance ramps) and urban arterials (i.e., driveways, cross streets and signalized intersections). The following report was developed to suggest techniques for estimating capacity and level of service on frontage roads.
Kay Fitzpatrick, R. Lewis Nowlin, and Angelia H. Parham. Procedures to Determine Frontage Road Level of Service and Ramp Spacing. Research Report 1393-4F, Texas Department of Transportation, Texas Transportation Institute, 1996.
Research Report 1393-4F contains procedures for the following:
- determining level of service on a continuous frontage road section
- analyzing frontage road weaving sections
- determining spacing requirements for ramp junctions.
Frontage Road Design Criteria
Design criteria for urban frontage roads are shown in Table 3-1: Geometric Design Criteria for Urban Streets using the collector criteria. Design criteria for suburban frontage roads are shown in Table 3-5: Geometric Design Criteria for Suburban Roadways using the collector criteria. Design criteria for rural frontage roads are shown in Table 3-19: Design Criteria for Rural Frontage Roads. Horizontal clearances are given in Table 2-11: Horizontal Clearances.
Any frontage road constructed will be designed to provide one-way operation initially. There may be exceptions in certain isolated instances; however, such exceptions will be considered only where, due to extraordinary circumstances, a one-way pattern would impose severe restrictions on circulation within an area. In those cases where such exceptions are considered, they must be approved by the Design Division at the schematic stage.
|
(US Customary) |
||||
|---|---|---|---|---|
|
Design Speed2 (mph) |
Min. Width1 for Future Traffic Volume of |
|||
|
- |
0-400 ADT |
400-1,500 ADT |
1,500-2,000 ADT |
2,000 or more ADT |
|
LANES (ft) |
||||
|
20 |
10 |
10 |
11 |
12 |
|
25 |
10 |
10 |
11 |
12 |
|
30 |
10 |
10 |
11 |
12 |
|
35 |
10 |
10 |
11 |
12 |
|
40 |
10 |
11 |
11 |
12 |
|
45 |
10 |
11 |
11 |
12 |
|
50 |
10 |
11 |
12 |
12 |
|
55 |
10 |
11 |
12 |
12 |
|
60 |
11 |
11 |
12 |
12 |
|
65 |
11 |
11 |
12 |
12 |
|
70 |
11 |
11 |
12 |
12 |
|
75 |
11 |
12 |
12 |
12 |
|
80 |
11 |
12 |
12 |
12 |
|
SHOULDERS (ft)4 |
||||
|
Each Shoulder Two-Way Operation |
23 |
4 |
8 |
8 – 10 |
|
Inside Shoulder One-Way Operation |
23 |
23 |
4 |
44 |
|
Outside Shoulder One-Way Operation |
23 |
4 |
8 |
8 – 10 |
|
1May retain existing paved width on a reconstruction project if total paved width is 24 ft and operating satisfactorily. 2Use rural collector criteria ( Table 3-6) for determining minimum design speed. 3At locations where roadside barriers are provided, use minimum 4 ft offset from travel lane edge to barrier face. 4If the one-way frontage road section contains three or more travel lanes, then minimum inside shoulder width is 8 – 10 ft. |
||||
|
(Metric) |
||||
|---|---|---|---|---|
|
Design Speed2 (km/h) |
Min. Width1 for Future Traffic Volume of |
|||
|
- |
0-400 ADT |
400-1,500 ADT |
1,500-2,000 ADT |
2,000 or more ADT |
|
LANES (m) |
||||
|
30 |
3.0 |
3.0 |
3.3 |
3.6 |
|
40 |
3.0 |
3.0 |
3.3 |
3.6 |
|
50 |
3.0 |
3.0 |
3.3 |
3.6 |
|
60 |
3.0 |
3.3 |
3.3 |
3.6 |
|
70 |
3.0 |
3.3 |
3.3 |
3.6 |
|
80 |
3.0 |
3.3 |
3.6 |
3.6 |
|
90 |
3.0 |
3.3 |
3.6 |
3.6 |
|
100 |
3.3 |
3.3 |
3.6 |
3.6 |
|
110 |
3.3 |
3.3 |
3.6 |
3.6 |
|
120 |
3.3 |
3.6 |
3.6 |
3.6 |
|
130 |
3.3 |
3.6 |
3.6 |
3.6 |
|
SHOULDERS (m)4 |
||||
|
Each Shoulder Two-Way Operation |
0.63 |
1.2 |
2.4 |
2.4 – 3.0 |
|
Inside Shoulder One-Way Operation |
0.63 |
0.63 |
1.2 |
1.24 |
|
Outside Shoulder One-Way Operation |
0.63 |
1.2 |
2.4 |
2.4 – 3.0 |
|
1May retain existing paved width on a reconstruction project if total paved width is 7.2 m and operating satisfactorily. 2Use rural collector criteria ( Table 3-6) for determining minimum design speed. 3At locations where roadside barriers are provided, use minimum 1.2 m offset from travel lane edge to barrier face. 4If the one-way frontage road section contains three or more travel lanes, then minimum inside shoulder width is 2.4 – 3.0 m. |
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Anchor: #i1084771
Conversion of Frontage Roads from Two-Way to One-Way Operation
Existing frontage roads in some areas are currently operating as two-way facilities. Such two-way operation has the following disadvantages:
- Higher crash rates are normally experienced when the frontage roads are two-way. In large part, this is because of the risk of essentially head-on collisions at the ramp terminals.
- Increased potential for wrong-way entry to the mainlanes.
- The intersections of the frontage roads with the arterials are much more complicated. Left turns from the arterial onto the frontage road must be accommodated from both directions. Accordingly, the signal phasing and sequencing options normally available at signalized diamond interchanges cannot be used.
- The overall traffic-carrying capacity of the frontage
roads is substantially less than if the same facility were re-striped
for one-way operation.
Existing two-way frontage roads should be converted to one-way operation when one or more of the following conditions occur.
- Queuing on the frontage road approach routinely backs up from the arterial intersection to within 100 ft. of a freeway entrance or exit ramp gore.
- The level-of-service of a signalized intersection of the frontage road and the arterial drops below level-of-service C.
- Queuing in the counter-flow direction (i.e. that which would not exist if the frontage road were one-way) routinely backs up from the stop line at a freeway entrance or exit ramp to within 100 ft. of the arterial street.
- Accident rate comparisons are above the statewide average accident rate for two-way frontage roads.
- Major freeway reconstruction or rehabilitation is occurring in a developed or developing area.
Conversion of two-way frontage roads located in urbanizing rural areas, where distances between crossover interchanges are relatively long, will require consideration of additional crossovers to minimize the distance traveled for adjacent residents and business patrons. The existence of an adequate local street system in the area will also facilitate traffic circulation and minimize the travel time impact of converting frontage roads from two-way to one-way operation.
The simple conversion of two-way to one-way frontage roads will be accomplished with ramp and terminal design based on reconstruction criteria shown in Chapter 3, Section 6, Freeways, Frontage Roads, while the balance of the existing frontage road lanes may retain dimensions that meet rehabilitation criteria shown in Chapter 4, Section 4, Frontage Roads. However, if the frontage roads are being reconstructed, then reconstruction design criteria shown in Chapter 3, Section 6, Freeways, will be applicable throughout the section.
Anchor: #CHDDGHBEInterchanges
The decision to develop a facility to freeway standards becomes the warrant for providing highway grade separations or interchanges at the most important intersecting roadways (usually arterials and some collectors) and railroads. A grade separation refers to the crossing of two roadways by a physical separation so that neither roadway interferes with the other. An interchange is a grade separation with connecting roadways (ramps, loops, or connections) that move traffic between the intersecting highways.
Effect on community. An interchange or series of interchanges on a freeway through a community may affect large continuous areas or even the entire community. For this reason, interchanges must be located and designed so that they will provide the best possible traffic service. Drivers who have exited from a freeway expect to be able to re-enter in the same vicinity; therefore, partial interchanges that do not serve all desired traffic movements should be avoided.
Classifications. Interchanges are classified in a general way, according to the number of approach roadways or intersection legs, as 3-leg, 4-leg and multi-leg interchanges. Through common usage, interchanges are descriptively called “Tee” (or Trumpet) for 3-leg design. Cloverleaf (full or partial) and Diamond for 4-leg, and Directional interchanges with three or more legs including direct connectors.
The following subsections include a brief description and some of the advantages and disadvantages of each of the following types of interchanges:
Anchor: #CHDIFIGGThree Leg Interchanges
Three-leg interchanges can take any of several forms, although all of the forms provide connections for the three intersecting highways. Three-leg interchanges should be used only after careful consideration because expansion to include a fourth leg is usually very difficult. If the potential exists that a fourth leg will ultimately be included, another type of interchange may be appropriate.
Trumpet. The most widely used 3-leg interchange is the trumpet type, as shown in Figure 3-17. This type of interchange is particularly suitable for the connection of a major facility and a freeway. Preference should be given to the major turning movements so that the directional roadway handles higher traffic volume and the loop the lower traffic volume.
Figure 3-17. Trumpet Three Leg Interchange.
Direct. High-type directional three-leg interchanges are those in which all movements are provided without the use of loops. These interchanges should be used only where all movements are large. They contain more than one structure or, alternatively, a three-level structure. Both variations are illustrated in Figure 3-18.
Figure 3-18. Directional Three Leg Interchange.
Anchor: #CHDBIFJEFour Leg Interchanges
Four-leg interchanges can take a wide variety of forms. The choice of interchange type is generally established after careful consideration of dominant traffic patterns and volumes, ROW requirements, and system considerations. The three primary types of four-leg interchanges are as follows:
Anchor: #CHDECBJJDiamond Interchanges
The diamond interchange is the most common interchange, especially in urban areas, since it requires less area than any other type. The diamond interchange is used almost exclusively for major-minor crossings since left-turn movements are made at-grade across conflicting traffic on the minor road. Separation between frontage road intersections in diamond interchanges in urban or suburban conditions should be 300 ft [90 m] as a minimum, as shown in Figure 3-19.
Figure 3-19. Typical Interchange For At-Grade Portion Of Diamond Interchange In Urban Or Suburban Areas. Click here to see a PDF of the image.
The diamond interchange may have several different configurations, as discussed in the following paragraphs and shown in Figure 3-20:
Conventional diamond without frontage roads. The conventional diamond (Figure 3-20 A) is the most common application of a diamond interchange. Traffic exits in advance of and near the cross street. Entering vehicles quickly access the freeway beyond or past the cross street. Its disadvantages include exiting vehicles backing up onto the freeway when long queues form on the ramp.
Conventional diamond with frontage roads. The conventional diamond with frontage roads (Figure 3-20 B) is a common variation of a diamond interchange. Traffic exits in advance of and near the cross street. Entering vehicles quickly access the freeway past the cross street. Its disadvantages include 1) exiting vehicles backing up onto the freeway when long queues form on the ramp or frontage road, and 2) most vehicles must go through the intersection to gain access to most frontage road property.
Reverse diamond or x- pattern. The reverse diamond or “X” interchange pattern (Figure 3-20 C) has primary application to locations with significant development along the frontage road. It provides access between interchanges and exiting queues do not back up onto the freeway. However, entering vehicles may have to accelerate on an upgrade and exiting maneuvers occur just beyond the crest vertical curve where weaving also takes place. The “X" ramp pattern also encourages frontage road traffic to bypass the frontage road signal and weave with the mainlane traffic. The “X” ramp pattern may cause some drivers to miss an exit located well in advance of the cross street.
Spread diamond. The spread diamond (Figure 3-20 D) involves moving the frontage roads outward to provide better intersection sight distance at the cross street and improved operational characteristics with signalized intersections, due to the separation between intersections. However, more additional right-of-way is required, which may limit its usage.
Stacked diamond. Sometimes access to and from the mainlanes is needed on two closely-spaced cross streets. Insufficient distance for consecutive entrance and exit ramps can be resolved by using grade separated ramps, resulting in a “stacked diamond” (Figure 3-20 E).
Split diamond. In some locations, it may be feasible and desirable to “split” the diamond by having one-way streets for the arterial movement (Figure 3-20 F). (This is especially true near central business districts where one-way street systems are common.) However, the split diamond can also be used to accommodate two closely-spaced two-way arterial roadways crossing a freeway.
Figure 3-20. Typical Diamond Interchanges.
Three level diamond. In urban areas, where the cross street carries a high volume of traffic, the three-level diamond interchange, illustrated in Figure 3-21, may be warranted. The through movements of both the controlled access facility and the cross street flow is uninterrupted with only the turning movements requiring regulation by stop signs or traffic lights. This type interchange is not usually recommended for use as the ultimate design at the crossing of two controlled access facilities since it requires left-turn interchanging traffic to negotiate three traffic signals or stop controls. However, as stage construction for a fully directional interchange between two controlled access facilities, the three-level diamond can be effective.
Figure 3-21. Three Level Diamond Interchange.
Single point diamond. A special type of freeway-to-arterial interchange has received attention during recent years and is worthy of discussion. AASHTO’s A Policy on Geometric Design of Highways and Streets refers to it as a “single point diamond” or “single point urban” interchange. In this type of interchange, the freeway mainlanes may go either over or under the crossing arterial and the turn movements occur at-grade on the arterial, as illustrated in Figure 3-22. This type of interchange has application only in specialized locations. Traffic operations and signalization must be carefully modeled prior to final design selection of the single point urban interchange.
Figure 3-22. Single Point Diamond Interchange.
Three level stacked diamond. The three-level stacked diamond interchange is also an interchange requiring only one signalized intersection. In a sense, it is a three-level version of the “single point diamond” configuration, as illustrated in Figure 3-23. This design grade separates both roadways, and accommodates turning movements with signal operations requiring only one signalized intersection. The two-phase signal operation at the intersection typically provides a level of throughput on the turning movements between a conventional diamond interchange and a fully directional interchange. Furthermore, it works best at separating high arterial cross-street and freeway traffic. It has the same shortcomings as the “single point diamond” in the way it brings the left turn movements together.
Figure 3-23. Three Level Stacked Diamond Interchange (see Figure 3-24 for At-Grade Portions of the Interchange).
As indicated in Figure 3-24, vehicles enter the intersection with oncoming vehicles to the right in contrast to the left as is the case on conventional diamond interchange intersections. Also, the design is less attractive with continuous frontage roads.
Figure 3-24. Three Level Stacked Diamond At-Grade Interchange.
Anchor: #CHDICAFDCloverleaf Interchanges
Cloverleaf interchanges are very common in many states. These types of interchanges were popular in the early era of freeway construction, but are usually no longer considered preferable for freeway to freeway movement, especially when interchange volumes are high. However, in some instances they may be appropriate when interchanging a freeway with a non-controlled access facility in a location away from an urban or urbanizing area. Cloverleafs should not be used where left-turn volumes are high (exceed 1200 pcph) since loop ramps are limited to one lane of operation and have restricted operating speeds.
Primary disadvantages of the cloverleaf design include the following:
- large right-of-way requirements
- capacity restrictions of loops, especially if truck volumes are significant
- short weaving length between loops
- trucks have difficulty with weaves and acceleration
When used, cloverleaf designs should include collector-distributor roads to provide more satisfactory operations as further noted in the section on Collector-Distributor Roads.
Full cloverleaf. The four-quadrant, full cloverleaf, illustrated in Figure 3-25, eliminates all left-turn conflicts through construction of a two-level interchange.
Figure 3-25. Full Cloverleaf Interchange.
Partial cloverleaf. A cloverleaf without ramps in all four quadrants, illustrated in Figure 3-26, is sometimes used when site controls (such as railroads or streams running parallel to the crossroad) limit the number of loops and/or the traffic pattern is such that the left-turn conflicts caused by the absence of one or more loops are within tolerable limits. With such an arrangement, left-turn conflicts at the ramp intersections require that satisfactory approach sight distance be provided. Several variations on partial cloverleafs are also discussed in the AASHTO’s A Policy on Geometric Design of Highways and Streets.
Figure 3-26. Partial Cloverleaf Interchange.
Anchor: #CHDIHHBFDirectional Interchanges
Interchanges that use direct or semi-direct connections for one or more left-turn movements are called “directional” interchanges (Figure 3-27). When all turning movements travel on direct or semi-direct ramps or direct connections, the interchange is referred to as “fully directional”. These connections are used for important turning movements instead of loops to reduce travel distance, increase speed and capacity, reduce weaving and avoid loss of direction in traversing a loop. “Fully directional” interchanges are usually justified at the intersection of two freeways.
Figure 3-27. Four Level Fully Directional Interchange Without Frontage Roads.
Four level without frontage roads. The four-level directional interchange as depicted in Figure 3-27 includes direct connections for all freeway-to-freeway movements, without continuation of any frontage roads through the interchange.
Five level with frontage roads. In some instances, it may be desirable to continue the frontage roads through the interchange at the first or second level, producing a five-level directional interchange. Where frontage roads are made continuous through the interchange, the lower three levels are a three-level diamond configuration. Where stage construction is desired, the three-level diamond will adequately serve moderate traffic volumes until the upper two levels of direct connections are constructed to complete the five-level interchange. Figure 3-28 depicts a five level interchange with frontage roads.
Figure 3-28. Five Level Fully Directional Interchange with Frontage Roads.
Anchor: #i1085186Ramps and Direct Connections
This subsection discusses ramps and direct connections and includes information on the following topics:
- General Information
- Design Speed
- Horizontal Geometrics
- Distance Between Successive Ramps
- Cross Section and Cross Slopes
- Sight Distance
- Metered Ramps
General Information
All ramps and direct connections should be designed for one-lane operation with provision for emergency parking; however, if the anticipated volume exceeds the capacity of one freeway lane, two-lane operation may be provided with consideration given to merges and additional entry lanes downstream. Several examples of ramps and connecting roadway arrangements are shown in Figures 3-29 through 3-35.
Figure 3-29. (US). Entrance/Exit Ramps For One-Way Frontage Roads. Click US Customary or Metric to see a PDF of the image.
Figure 3-30. (US). Entrance Or Exit Ramps For Two-Way Frontage Roads (Turnaround Provided). Click US Customary or Metric to see a PDF of the image.
Figure 3-31. (US). Two-Way Frontage Roads Exit and Entrance Ramps (Turnaround Prohibited). Click US Customary or Metric to see a PDF of the image.
Figure 3-32. (US). Design Details For Ramp Transitions Into Single or Multiple Roadways. Click US Customary or Metric to see a PDF of the image.
Figure 3-33. (US). Typical Exit Ramps Without Frontage Roads. Click US Customary or Metric to see a PDF of the image.
Figure 3-34. (US). Typical Channelized Exit and Entrance Ramps (Two-Way Frontage Road). Click US Customary or Metric to see a PDF of the image.
Figure 3-35. (US). Design Details for One-Lane and Two-Lane Ramps or Direct Connectors. Click US Customary or Metric to see a PDF of the image.
Once ramps have been located on a schematic layout and the same has been exhibited at a public hearing or the design has otherwise become a matter of public record, extreme caution should be exercised in making any subsequent changes in ramp location to better serve areas that may have developed after the original design was determined. In all cases, proposed changes should be submitted to the Design Division, and another public hearing may be required.
Right-side ramps are markedly superior in their operational characteristics and safety to those that leave or enter on the left. With right-side ramps, merging and diverging maneuvers are accomplished into or from the slower moving right travel lane. Since a high majority of ramps are right-side, there is an inherent expectancy by drivers that all ramps will be right-side, and violations of driver expectancy may adversely affect operation and safety characteristics.
Direct access to and from ramps or direct connections can seriously impair safety and traffic operations and, therefore, should not be permitted.
Anchor: #CHDCAGIEDesign Speed
There should be a definite relationship between the design speed on a ramp or direct connection and the design speed on the intersecting highway or frontage road. All ramps and connections should be designed to enable vehicles to leave and enter the traveled way of the freeway at no less than 50 percent (70 percent usual, 85 percent desirable) of the freeway’s design speed. Table 3-20 shows guide values for ramp/connection design speed. The design speed for a ramp should not be less than the design speed on the intersecting frontage roads. AASHTO’s A Policy on Geometric Design of Highways and Streets provides additional guidance on the application of the ranges of ramp design speed shown in Table 3-20:
|
(US Customary) |
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|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Highway Design Speed (mph) |
30 |
35 |
40 |
45 |
50 |
55 |
60 |
65 |
70 |
75 |
80 |
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|
Ramp** Design Speed (mph): |
- |
||||||||||||||||||
|
Upper Range (85%) |
25 |
30 |
35 |
40 |
45 |
48 |
50 |
55 |
60 |
65 |
70 |
||||||||
|
Mid Range (70%) |
20 |
25 |
30 |
33 |
35 |
40 |
45 |
45 |
50 |
55 |
60 |
||||||||
|
Lower Range (50%) |
15 |
18 |
20 |
23 |
25 |
28 |
30 |
30 |
35 |
40 |
45 |
||||||||
|
(Metric) |
|||||||||||||||||||
|
Highway Design Speed (km/h) |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
120 |
130 |
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|
Ramp** Design Speed (km/h): |
- |
||||||||||||||||||
|
Upper Range (85%) |
40 |
50 |
60 |
70 |
80 |
90 |
100 |
110 |
120 |
||||||||||
|
Mid Range (70%) |
30 |
40 |
50 |
60 |
60 |
70 |
80 |
90 |
100 |
||||||||||
|
Lower Range (50%) |
20 |
30 |
40 |
40 |
50 |
50 |
60 |
70 |
80 |
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|
* For corresponding minimum radius, see Table 2-6. **Loops: Upper and middle range values of design speed generally do not apply. The design speed on a loop should be no less than 25 mph [40 km/h] (185 ft [55 m] minimum radius) based on an emax of 6%. Particular attention should be given to controlling superelevation on loops due to the tight turning radii and speed limitations. |
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Anchor: #CHDCJEFG
Horizontal Geometrics
Lane and shoulder widths for ramps and direct connections are shown in Table 3-18.
Figure 3-36 provides design criteria for entrance and exit ramp acceleration, deceleration, and taper lengths; adjustment factors for grade effects are shown in Table 3-14: Speed Change Lane Adjustment Factors as a Function of a Grade
Exit and entrance ramp typical details are shown in Figure 3-23, Figure 3-24, Figure 3-25, Figure 3-26, Figure 3-27, Figure 3-28, and Figure 3-29.
Channelized (braided) entrance and exit ramps, as typified in Figure 3-28, should be used only where ramp volumes are considerably greater than frontage road traffic such as where stub frontage roads occur. Where used, the exit ramp desirably should cross the frontage road at approximately 90 degrees to minimize wrong-way entry. Passing should be restricted between the crossroad and the channelized area.
Figure 3-36. (US). Lengths of Exit and Entrance Ramp Speed Change Lanes. Click US Customary or Metric to see a PDF of the image.
Anchor: #CHDDICBHDistance Between Successive Ramps
The minimum acceptable distance between ramps is dependent upon the merge, diverge and weaving operations that take place between ramps as well as distances required for signing. For analysis of these requirements, see the Highway Capacity Manual. Figure 3-37 shows minimum distances between ramps for various ramp configurations.
Figure 3-37. Arrangements For Successive Ramps. Click here to see a PDF of the image.
Anchor: #CHDGGFFHCross Section and Cross Slopes
Superelevation rates, as related to curvature and design speed of the ramp or direct connector, are given in Table 3-21. While connecting roadways represent highly variable conditions, as high a superelevation rate as practicable should be used, preferably in the upper half or third of the indicated range, particularly in descending grades. Superelevation rates above 8% are shown in Table 3-21 only to indicate the limits of the range. Superelevation rates above 8% are not recommended and a larger radius is preferable.
|
(US Customary) |
||||||
|---|---|---|---|---|---|---|
|
Radius (ft) |
Range in Superelevation Rate (percent) For Connecting Roadways With Design Speed, mph, of: |
|||||
|
- |
20 |
25 |
30 |
35 |
40 |
45 |
|
90 |
2-10 |
- |
- |
- |
- |
- |
|
150 |
2-8 |
4-10 |
- |
- |
- |
- |
|
230 |
2-6 |
3-8 |
6-10 |
- |
- |
- |
|
310 |
2-5 |
3-6 |
5-9 |
8-10 |
- |
- |
|
430 |
2-4 |
3-5 |
4-7 |
6-9 |
9-10 |
- |
|
540 |
2-3 |
3-5 |
4-6 |
6-8 |
8-10 |
10-10 |
|
600 |
2-3 |
2-4 |
3-5 |
5-7 |
7-9 |
8-10 |
|
1000 |
2 |
2-3 |
3-4 |
4-5 |
5-6 |
7-9 |
|
1500 |
2 |
2 |
2-3 |
3-4 |
4-5 |
5-6 |
|
2000 |
2 |
2 |
2 |
2-3 |
3-4 |
4-5 |
|
3000 |
2 |
2 |
2 |
2 |
2-3 |
3-4 |
|
* See Tables 2-6 and 2-7 for design speeds greater than 45 mph. |
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|
(Metric) |
||||||
|
Radius (m) |
Range in Superelevation Rate (percent) For Connecting Roadways With Design Speed, km/h, of: |
|||||
|
- |
20 |
30 |
40 |
50 |
60 |
70 |
|
15 |
2-10 |
- |
- |
- |
- |
- |
|
25 |
2-9 |
2-10 |
- |
- |
- |
- |
|
50 |
2-8 |
2-8 |
4-10 |
- |
- |
- |
|
80 |
2-6 |
2-6 |
3-8 |
6-10 |
- |
- |
|
100 |
2-5 |
2-4 |
3-6 |
5-8 |
- |
- |
|
115 |
2-3 |
2-4 |
3-6 |
5-8 |
8-10 |
- |
|
150 |
2-3 |
2-3 |
3-5 |
4-7 |
6-8 |
- |
|
160 |
2-3 |
2-3 |
3-5 |
4-7 |
6-8 |
10-10 |
|
200 |
2 |
2-3 |
2-4 |
3-5 |
5-7 |
6-8 |
|
300 |
2 |
2-3 |
2-3 |
3-4 |
4-5 |
5-7 |
|
500 |
2 |
2 |
2 |
2-3 |
3-4 |
4-5 |
|
700 |
2 |
2 |
2 |
2 |
2-3 |
3-4 |
|
1000 |
2 |
2 |
2 |
2 |
2 |
2-3 |
|
* See Tables 2-6 and 2-7 for design speeds greater than 70 km/h. |
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The cross slope on portions of connecting roadways or ramps on tangent normally is sloped one way at a practical rate of 1.5% to 2%.
The change in pavement edge elevation per given length of connecting roadway or ramp should be that as shown in Table 2-8: Maximum Relative Gradient for Superelevation Transition. The maximum algebraic difference in pavement cross slope at connecting roadways or ramps should not exceed that set forth in Table 3-22:
|
(US Customary) |
(Metric) |
||
|---|---|---|---|
|
Design Speed of Exit or Entrance Curve (mph) |
Maximum Algebraic Difference in Cross Slope at Crossover Line (%) |
Design Speed of Exit or Entrance Curve (km/h) |
Maximum Algebraic Difference in Cross Slope at Crossover Line (%) |
|
Less than or equal to 20 |
5.0 to 8.0 |
Less than or equal to 30 |
5.0 to 8.0 |
|
25 to 30 |
5.0 to 6.0 |
40 to 50 |
5.0 to 6.0 |
|
Greater than or equal to 35 |
4.0 to 5.0 |
Greater than or equal to 60 |
4.0 to 5.0 |
The cross section of a ramp or direct connector is a function of the following variables:
- number of lanes determined by traffic volume
- minimum lane and shoulder width
- lane balance
- where two lanes are required by volume, the provision of parallel merging two lanes onto the mainlanes must be provided at the terminal
Sight Distance
On all ramps and direct connections, the combination of grade, vertical curves, alignments and clearance of lateral and corner obstructions to vision shall be such as to provide sight distance along such ramps and connections from terminal junctions along the freeway, consistent with the probable speeds of vehicle operation. Sight distance and sight lines are especially important at merge points for ramps and mainlanes or between individual ramps. Table 2-1: Stopping Sight Distance shows recommended stopping sight distances for ramps and direct connections.
The sight distance on a freeway preceding the approach nose of an exit ramp should exceed the minimum stopping sight distance for the freeway design speed, preferably by 25 percent or more. Decision sight distance, as discussed in Decision Sight Distance in Chapter 2, is a desirable goal.
Anchor: #i1085605Grades and Profiles
Design controls for crest and sag vertical curves on ramps and direct connectors may be obtained from Figures 2-7 through 2-10. Longer vertical curves with increased stopping sight distances should be provided wherever possible.
The tangent or controlling grade on ramps and direct connectors should be as flat as possible, and preferably should be limited to 4 percent or less. AASHTO’s A Policy on Geometric Design of Streets and Highways has additional discussion on ramp gradients.
Anchor: #CHDEJABHMetered Ramps
Where ramps are initially, or subsequently, expected to accommodate metering, the geometric design features shown in Design Criteria for Ramp Metering may be considered. Ramp metering, when properly designed and installed, has been shown to have potential benefits for the operation of the mainlanes. However, since ramp meters are installed to control the number of vehicles that are allowed to enter the mainlanes, an analysis of the entire roadway network area should be done to determine any adverse operational impacts to other roadways. It is suggested that the analysis specifically include both frontage road and adjacent cross street operations of through traffic, turning movements, and queue lengths.
Anchor: #CHDCFJEECollector-Distributor Roads
A collector-distributor road may be warranted within an interchange, through two adjacent interchanges or continuous for some distance along the freeway through several interchanges. Collector-distributor roads should be provided at all cloverleaf interchanges and particularly at such interchanges on controlled access facilities. A collector-distributor road is designed to meet the following goals:
- transfer weaving from the mainlanes
- provide single-point exits from the main lanes
- provide exit from the main lanes in advance of cross roads.
Where there is considerable demand for frequent ingress and egress, as in and near the business district of large cities, a collector-distributor road, continuous for some distance, should be provided.
Anchor: #i1085663Frontage Road Turnarounds and Intersection Approaches
Turnaround lanes are to be provided at all interchanges with major arterials in urban and suburban areas where the freeway lanes are flanked by one-way frontage roads. Turnaround lanes are not to be provided where two-way frontage roads are used. In urban and suburban areas, overpasses should be arranged so that turnarounds may be added in the future. This includes provisions for end spans and vertical clearance for future turnarounds at overpasses. Underpass situations should also allow for vertical clearances on future elevated turnarounds.Figure 3-38 shows a typical turnaround at a diamond interchange.
When the cross street overpasses the freeway, the resulting turnarounds will be on bridge structures. In these cases, sight lines and distances should be carefully evaluated with respect to any bridge railing sight obstructions.
Figure 3-38. Typical Diamond Interchange With Frontage Road. Click here to see a PDF of the image.