234.3 Directional Interchanges
From Engineering Policy Guide
Advantages of Directional Interchanges
- Preferred configuration where two high-volume freeways intersect.
- Reduced travel distance on the ramps, increased ramp speed and capacity and elimination of weaving.
- Offers high capacity movements for both trough and turning traffic with comparatively little additional area needed for construction when compared to a cloverleaf interchange.
Disadvantages of Directional Interchanges
- More costly to construct than a cloverleaf interchange due to the increase number and length of ramps and the number of bridge crossings.
- Requires a large area of right of way.
- The configuration and design of each interchange is uniquely based on traffic volumes and patterns, environmental and cost considerations. Therefore, detailed and time-consuming studies of all likely alternatives are necessary.
Directional interchanges are generally preferred where two high-volume freeways intersect. They reduce travel distance, increase ramp speed and capacity, and eliminate weaving movements. Since traffic movements between the two freeways are free-flow with this interchange configuration, at-grade intersections are eliminated, only direct ramp connections from one freeway to the other. Fully directional interchanges are costly to construct due to the increased number and length of the ramps and the increased number of bridge crossings, but they offer high capacity movements for both through and turning traffic with comparatively little additional area needed for construction. The configuration and design of each interchange is uniquely based on factors such as traffic volumes, patterns, environmental considerations, and costs. Therefore, detailed and time-consuming studies are usually necessary for each proposed interchange and must include all likely alternatives.
There are many configurations for directional interchanges that use various combinations of directional, semi-directional, and loop ramps. Any one of them may be appropriate for a certain set of conditions, but only a limited number of patterns are generally used. The most common configurations fill the least space, minimize complex structures, minimize internal weaving and fit the common terrain and traffic conditions.
Weaving, left-side exits, and left-side entrances are undesirable within a directional interchange. However, there may be instances where they cannot be reasonably avoided because of site restrictions, costs or other considerations. With heavy left-turn movements, the terminals are to be designed as major forks and branch connections.
Access for all traffic movements at interchanges on the state highway system will be provided. In some cases, traffic volumes may not warrant the provision of all ramps at an interchange at the time of initial construction. However, when this occurs right of way will be purchased prior to initial construction that will provide for the “future ramps”. The plans for initial construction of the interchange will show the location of the “future ramps” that will provide for these traffic movements.
234.3.1 Ramp and Preliminary Plan Criteria
Traffic capacities for ramp design are subject to variation and are limited by the geometric features of the ramp itself, the ramp termini, the weaving sections and the volume of through traffic. An appropriate model, such as the Highway Capacity Manual, is used to determine these capacities. Additional information is contained in A Policy on Geometric Design of Highways and Streets (AASHTO Green Book).
Ramp base lines are always equated to the survey centerline and other ramp base line intersection points or the crossroad centerline intersection point. The equations include offsets and intersection angles.
Interchange ramps are numbered for ease of identification. Ramp details can be found on Standard Plan 203.40.
The geometric layout of a directional interchange is prepared by the district and includes a plan sheet and a profile grade sheet. This information is submitted to the Design Division for review and comment.
Drawings with detail interchange geometrics are developed to the same scale used on the plans and on standard size, 22 in. x 34 in. sheets. The drawing includes complete alignment details for ramp base lines and shows the limits of pavement. A supplemental standard-size full-profile sheet or cross-section sheet is used for plotting profiles and developing ramp grades. The profile is plotted to the horizontal scale used on the plans, and to a vertical scale of 1 in. = 10 ft. (1:100). If a standard-size sheet is too small, additional standard-size sheets are used, with match lines, or the layout may be prepared on a sheet of multiples of the standard size. The plan is developed to the best possible accuracy since this sheet becomes the base for later drawings (i.e. for grade separation reports). Grades are considered tentative at this stage and are developed in a similar manner to the preliminary plans. The plan sheet(s) includes complete traffic data for the interchange, including all turning movements. The traffic data is shown with the design hourly volume (DHV) over average annual daily traffic (AADT) and also the percent of trucks.
At this preliminary plan stage, two prints of the interchange geometric sheets are submitted to the Design Division for review and comment. Three prints are submitted for all interstate projects so approval from FHWA can also be obtained.
The ramp curvature at the ramp nose or gore area will have a minimum radius of 6 degrees or less. The delta for this curve will be as small as the ramp grade and location of the ramp terminal at the crossroad will permit. If practicable, the ramp curvature is established so both ends of the curve are at least 100 ft. from the ramp nose measured along the ramp base line.
Desirably, the anticipated posted speed on a ramp in a directional interchange will approximate the mainline low volume operating speed. This speed is not always practical; therefore, the desirable speed on the ramp is 0.7 of the mainline operating design speed, but not less than the values shown below. These values apply to the controlling ramp curve, usually on the ramp proper.
|HIGHWAY OPERATING SPEED, mph||RAMP OPERATING SPEED, mph|
|UPPER RANGE (85%)||MIDDLE RANGE (70%)||LOWER RANGE (50%)|
For anticipated operating speeds of more than 50 mph, the loop design speed will not be less than 25 mph, with a minimum loop radius of 150 ft. Anticipated loop ramp operating speeds above 25 mph in urban areas and 30 mph in rural areas are seldom practical. For a 5 mph increase in anticipated operating speed on loop ramps, the travel distance increases over 50 percent and the required right of way increases about 130 percent. If practicable, a speed change transition curve preferably 430 ft. in radius (or at least 230 ft. in radius) is used at the loop terminals, depending upon the corresponding mainline operating speed.
This radius curve is compounded with a shorter radius curve for the central portion of the loop. The speed transition curve extends at least 100 ft. beyond the ramp nose and begins or ends at least 50 ft. from the end of the grade separation structure. Minimum radii for turning speeds are shown below.
|MINIMUM RADII AND SUPERELEVATION FOR TURNING SPEEDS|
|OPERATING SPEED MPH||RADIUS FT.||SUPER ELEVATION FT./FT.||LENGTH OF CIRCULAR ARC, DESIRABLE FT.|
234.3.2 Basic Number of Lanes
The "basic number of lanes" is a fundamental design control maintained along a mainline roadway in addition to the requirements of lane balance and capacity. The basic number of lanes is the constant or minimum number of lanes provided throughout a significant length of the mainline roadway. This number is exclusive of the number of auxiliary lanes. Indiscriminate adding or dropping of basic lanes is discouraged.
234.3.3 Coordination of Lane Balance
Proper lane balance is maintained on the main-line roadway at interchanges. The required number of lanes as determined by volume-capacity relations sometimes changes significantly at the entrance or exit points of the interchange. The following requirements for lane balance can generally be applied to both entrance and exit ramp traffic.
- At entrance ramps, the number of lanes beyond the merging of two traffic streams is not less than the sum of all traffic lanes on the two merging roadways, minus one.
- At exit ramps, the number of approach lanes on the main-line roadway must be equal to or greater than the number of lanes on the mainline roadway beyond the exit plus the number of lanes on the exit ramp, less one.
Chapter 10 of the AASHTO Green Book has additional information concerning the coordination of lane balance and the basic number of lanes.
Major fork and branch connection designs with appropriate signing (see EPG 903 Highway Signing) to designate the optional exit lane destinations may be considered where appropriate as discussed in the AASHTO Green Book. Separate lanes with separate lane use signing are preferred where major forks occur.
234.3.4 Auxiliary Lanes for Acceleration and Deceleration (Parallel Type)
Minimum speed change lengths are given in Tables 10-3 and 10-5 of the AASHTO Green Book. Lengths shown in these tables are for grades of 2% or less on the speed change lane. Table 10-4 of the AASHTO Green Book provides adjustment to these lengths for grades over 2%. Speed change lanes are provided at all ramp entrances and exits where the number of through traffic lanes each side of the ramp terminal are equal. The combined length of full width acceleration lane and taper will not be less than 600 ft. A shoulder, at least 6 ft. wide, is provided for auxiliary lanes along the through traffic lanes (except a 4-foot shoulder will be provided along a median acceleration lane). Auxiliary lane width is the same as the width provided for the through traffic lanes. In rural areas, the ramp nose will be visible to approaching traffic for a distance equal to at least 1.25 times the stopping sight distance on the freeway
Where interchanges are closely spaced the auxiliary lane for acceleration will be extended to the exit of the next interchange. An entrance lane followed by a lane exiting forms a traffic weaving section that requires added pavement width and length for weaving capacity. The capacity of the auxiliary lane connecting the on-ramp with the off-ramp will be determined using the Highway Capacity Manual or an appropriate traffic-modeling program (i.e. VISSIM). The weaving section will have a length and number of lanes based on the appropriate level of service outlined in the Facility Selection article. Where a two-lane entrance ramp or a two-lane exit ramp is needed for capacity or lane balance, the effective length of auxiliary lane will be determined as illustrated in Figures 10-74 and 10-73 of the AASHTO Green Book.
234.3.5 Clear Zones
Clear zones will be provided on all ramps, where feasible. Specific guidance concerning clear zones is found in the Clear Zone article.
234.3.6 Lane Drop
Where a reduction in mainline traffic demand indicates a need for less traffic capacity, a lane drop or reduction in the number of through lanes is made on the exit ramp, preferably one with a high traffic volume. This reduction may be made provided the exit volume is sufficiently large to change the basic number of lanes beyond this point on the route as a whole. A lane drop is only made on a right hand exit ramp. Under no circumstance will a lane drop be made on a left hand exit ramp. This creates an unacceptable situation for safe traffic operation.
Where a lane drop or a reduction in the number of thruway lanes is made, an "escape lane" or a pavement taper of 50 to 1 convergence is provided beyond the gore nose for traffic to merge into the remaining through traffic lanes. Similarly, where two lane exit ramps are used, an "escape lane" is provided, if an auxiliary lane for lane balance is not provided beyond the gore nose.
Minimum controls for superelevation of short radius curves on ramps are shown in Table 2. A maximum super elevation rate of 6% is used (refer to Table 3, below) when a short radius curve is on a bridge structure. This is the maximum superelevation suitable for satisfactory traffic safety under snow and ice conditions. The maximum rate of cross slope change is 5% per 100 ft. to transition the superelevation cross slope back to normal cross slope. Ramp entrances and exits are designed to reach full superelevation at the ramp nose, if full superelevation can be obtained at this point. Superelevation transition for typical ramp entrances and exits are shown on standard plans. At ramp terminals with the thruway, the maximum algebraic difference in pavement cross slope is 5%.
|e (%)||Anticipated Posted Speed|
The general grade layout of interchanges, such as cross road over or under and ramp grades is selected and designed with grading economy in mind. The desirable maximum ramp gradient is 5 percent. In special cases, ramp grades as steep as 7 percent may be used. The use of grades steeper than 5 percent is usually restricted to short grades in urban or suburban areas. Vertical curves for ramps, both crest and sag, are designed to meet the requirements given in the Vertical Alignment article based on the desirable ramp operating speed, except passing sight distance is not considered. The ramp grades are also designed in accordance with these same requirements, regardless of whether the ramp grade is up or down in relation to the roadway. Mainline grades are set, if at all possible, prior to developing the geometrics and ramp grades. The best procedure is to develop grades through and adjacent to the interchange so mainline grades outside the interchange area can be adjusted to finally balance grading quantities, and the crossroad through the interchange can be adjusted as necessary to provide near minimum vertical clearance for the grade separation based upon the final structure layout. This procedure will cause a minimum of revision to the geometrics as the grades are finally adjusted.
MicroStation, GEOPAK or other computer programs or aids are used to compute grading quantities in interchange areas. The method used is dependent upon the type of interchange, the terrain, and other site-specific factors. Regardless of the method, the plans will include adequate cross sections or contours to determine the quantity of grading material within the interchange area.
234.3.10 Site Grading
Grading quantities in interchange areas Class A, Class C or Unclassified Excavation will include site grading necessary to properly handle drainage, to improve appearance, and to eliminate poor sight distance of other intersections. Where possible site grading is done for all interchange areas. Preliminary plans are reviewed carefully to determine if minimal site grading will address these items and yet provide economy in the design.
Where site grading of an interchange area is made, it is desirable to provide flat slopes and smooth contour lines. Slopes on the outside of ramps will not be steeper than 1:3 and carried back to its intersection with the mainline roadway.
With additional site grading and flatter slopes, guardrail would only be needed to protect bridge ends within the interchange area. The grading and slopes will also aid in reducing sign post size, length and location.
The plans will provide adequate facilities for handling drainage through and from the interchange area, including adequate provisions to prevent water or melting snow from running across pavements. Erosion control in the interchange areas is also provided in the design. Storm water detention basins will be considered within an interchange in urban areas.
234.3.12 Typical Sections
The plans will include detailed typical sections for all ramps and the crossroad not covered on the plans for the mainline roadway. They will also include the location of survey and base lines, and the location of the profile grade in relation to the typical section.
234.3.13 Contract Plans
Contract plans show complete details for the construction of interchanges, including grading, geometrics, paving and drainage. Interchange grading quantities are tabulated on the plans, separated as much as conveniently possible from the main roadway grading quantities. Interchange grading quantities are considered in determining balance points in and adjacent to the interchange area. Details for typical ramp intersections are shown on standard plans. Similar details are required for ramp intersections not covered by the standard plans.
234.3.14 Lighting, Signing and Pavement Marking
The placement and maintenance requirements of lighting, signs and pavement marking, must be considered early in the design process. Standard MoDOT equipment will be used.
- The most important design principles are uniformity of light and minimization of glare.
- The mainline and crossroads are well lighted.
- Good lighting is provided at the ramp junctions.
- Detailed information is contained in EPG 901 Lighting.
Appropriate signing is provided in the interchange area to avoid motorist confusion. Important points to consider are:
- Overhead guide signing is recommended for the approaching roadways
- Traffic guide sign applications on the exit ramps are to be consistent with mainline signing.
- Use advance signing such as lane use signs over each lane on the roadway approaching the interchange. The sign support structure is placed at or just beyond the point where the turn lane is fully developed.
- Detailed information is contained in EPG 903 Signing
- Appropriate pavement marking is provided in the interchange to avoid motorist confusion.
- Detailed information is contained in EPG 620.2 Pavement and Curb Markings.
234.3.15 Maintenance and Emergency Crossovers
- facilitate maintenance activities such as snow removal and
- provide ample access for law enforcement or emergency medical services responding to roadway incidents.
Maintenance crossovers may be needed at one or both ends of interchange facilities, depending on the type of interchange.
Information concerning their location and requirements for construction can be found at EPG 240 Maintenance and Emergency Crossovers.