234.4 Single Point Urban Interchanges (SPUIs)
234.4.1 Discussion
A Single Point Urban Interchange (SPUI) is typically characterized by narrow right-of-way, high construction costs, and greater traffic capacity than conventional tight diamond interchanges. SPUIs usually function better without frontage roads. They are primarily suited for urban areas where right-of-way is restricted and expensive. Primary features of a SPUI are all four turning movements can be controlled by a single traffic signal. They may be selected as the best alternative for an improvement instead of a diamond interchange when the total right-of-way and construction costs are evaluated for each interchange type used to address traffic capacity.
234.4.1.1 Advantages
A SPUI offers several advantages including potentially significant right-of-way cost reductions by constructing it in a relatively narrow right-of-way. The primary operational advantage of this interchange configuration is the paths of vehicles making opposing left turns do not intersect. In addition, the right-turn movements are typically free-flow movements and only the left turns must pass through the signalized intersection. As a result, a major source of traffic conflict is eliminated, thus increasing overall intersection efficiency and reducing the signal operation needed from four-phases to three. Since the SPUI has only one intersection, as opposed to two for a diamond interchange, the operations of the single traffic signal may result in reduced delay through the intersection area when compared to a diamond interchange when turning movements are balanced. Radii for left-turn movements through the intersection are significantly larger than at conventional intersections so the left turns can move at higher speeds. These operational improvements may result in a higher capacity than a conventional tight diamond interchange.
| Interchange, Single Point |
| Report 2004 |
| Summary 2005 |
| See also: Innovation Library |
234.4.1.2 Disadvantages
The primary disadvantage of a SPUI is the high construction costs associated with bridges. A SPUI with ramp intersections under the bridge need long bridges to span the large intersection below. A two-span structure is not a design option because a center column would conflict with traffic movements. Single-span overpass bridges are typically 220 ft in length. A SPUI with ramp intersections on the bridge tend to be very wide resulting in high costs. Where right-of-way is tightly constrained, a SPUI will typically utilize extensive retaining walls, further adding to the cost. However, the higher construction cost of a SPUI is often offset by the reduced right-of-way cost.
A second potential problem encountered with a SPUI is the length and geometry of the path for left-turning vehicles through the intersection. This length requires a longer clearance interval that leads to longer delays at the traffic signal. Like most intersections, left-turning vehicles pass to the left of opposing left-turning vehicles. However, due to the size and distance between opposing approaches, the path of left-turning vehicles does not resemble a quarter of a circle found at typical intersections, but rather resembles a quarter of an ellipse. To provide positive guidance for this non-traditional path, various features have been developed. At a minimum, white extension lines are applied through the intersection.
The skew angle between the two roadway alignments has an adverse effect on SPUIs because it increases clearance distances and adversely affects sight distance. Severely skewed alignments may also increase the length of the bridge and widen the distance between the stop bars on the local streets. Extreme care should be exercised in planning to use a SPUI when the skew angle approaches 30 degrees.
234.4.1.3 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, the volume of through and turning traffic and intersection spacing within the functional area of the interchange. Access Management Guidelines are used for additional information. Procedures and methods for evaluating these capacities are given in the Highway Capacity Manual and 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 district approves the geometric layout of a SPUI from the schematic drawing shown on the preliminary plan.
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.
234.4.1.4 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.
234.4.1.5 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.
234.4.1.6 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.
234.4.1.7 Clear Zones
Clear zones will be provided on all ramps, where feasible. Specific guidance concerning clear zones is found in the Clear Zone article.
234.4.1.8 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.
234.4.1.9 Superelevation
Minimum controls for superelevation of short radius curves on ramps 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. |
| 15 | 50 | 0.00 | 60 |
| 20 | 90 | 0.02 | 60 |
| 25 | 150 | 0.04 | 70 |
| 30 | 230 | 0.06 | 110 |
| 35 | 310 | 0.08 | 140 |
| 40 | 430 | 0.08 | 190 |
| 45 | 540 | 0.08 | 200 |
A maximum super elevation rate of 6% is used 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%.
234.4.1.10 Grades
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 for diamond-type interchanges at the ramp intersection with the crossroad are designed in accordance with these same requirements, regardless of whether the ramp grade is up or down in relation to the crossroad. Crossroad grades, ramp grades, and sight distances in the vicinity of diamond-type ramp intersections with the crossroad are developed in accordance with the requirements given in the At-Grade Intersections article. If the crossroad operating speed is unknown, or is less than 30 mph, an operating speed of 30 mph is used. 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.
234.4.1.11 Grading
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.
234.4.1.12 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.
234.4.1.13 Drainage
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.
234.4.1.14 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.
234.4.1.15 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.
234.4.1.16 Basic Considerations
Several basic design considerations can optimize the geometrics and operation of a SPUI. First, it is desirable that the left-turn curve be a single radius. This will, however, typically increase the amount of right of way required for the SPUI and/or require a larger bridge structure. Where it is not practical to provide a single radius, and curves are compounded from a larger to a smaller radius, the smaller radius should be at least half the radius of the larger. Another important design feature is to provide stopping sight distance on the left-turn movements equal to or exceeding that required for the design speed of the radius involved. A third design feature that can improve intersection operation is to provide additional median width on the cross street. The stop bar location on the cross street is dependent on the wheel tracks from the opposing ramp left-turn movement. By widening the median, the stop bar on the cross street can be moved forward, thus reducing the size of the intersection and the distance each vehicle travels through the intersection. The results include greater available green time for the signal and less potential driver confusion from an expansive intersection area. Avoid placing a SPUI on a crest vertical curve. Doing so makes it difficult for a driver to determine the proper path through the interchange. It is also important to ensure the adjoining intersection on the crossroad can adequately handle the increased traffic generated by the SPUI. Lack of capacity at this intersection can reduce the efficiency of the SPUI.
A SPUI combined with a one-way frontage road will reduce the efficiency of the interchange. Frontage roads, if necessary, should be one-way in the direction of the ramp traffic. A slip ramp from the mainline to the frontage road provides access to and from the intersection. This ramp should connect to the frontage road at least 650 ft, and preferably greater than 1,000 ft, from the crossroad. The traffic signal needs a fourth phase to provide through movements on the frontage roads. A free-flow, U-turn movement may be desirable to expedite movements from one direction on the frontage road to the other.
Right-turn lanes at SPUIs are typically separated from the left-turn lanes, often by a considerable distance. The exit ramp right turn can be a free or controlled movement. The design of free right turns should include an additional lane on the cross street beginning at the free right-turn lane and continuing for at least 200 ft before merging. Free-flow right turns from the exit ramp to an arterial crossroad are not desirable when the nearest intersection on the crossroad is within 500 ft because there may be inadequate weaving distance between the exit ramp and the adjacent intersection. When a yield sign or a traffic signal controls the right turn movement, adequate right-turn storage on the exit ramp should be provided to prevent blockage of vehicles turning left or traveling straight. Free-flow right turns on entrance ramps pose little operational problems, assuming adequate merge length is provided on the entrance ramp. The right-turn lane on the entrance ramp should extend at least 100 ft beyond the convergence point before beginning the merge.
234.4.1.17 Pedestrians
Pedestrian movement through a SPUI can result in a loss of traffic capacity, as well as posing a risk to the pedestrian’s safety. If the movement of pedestrians through an interchange is critical, then the use of a SPUI must be very carefully considered. Pedestrian crossing of the local street at ramp terminals will require an additional signal phase, resulting in reduced operational efficiency. Therefore, the overall design may include provision of pedestrian crossings at adjacent intersections. A pedestrian bridge near the SPUI may also be an acceptable alternative. Pedestrian movements parallel to the local street are more readily handled. If, however, crosswalks are provided at ramps, they should be perpendicular to the ramp direction of travel and near to the local street. Perpendicular crosswalks minimize the length of the crossing, minimize conflicting movements, and reduce the amount of signal time needed for the pedestrian movement . Crosswalks located near the local street meet driver expectation and allow good sight distance to the pedestrian crossing.
234.4.1.18 Specific Design Criteria
The following criteria for various elements of a SPUI serve as a guide for their design.
Bridge:
- Overpass should carry the freeway (major roadway)
- Typical bridge span length is 120 to 200 ft.
Traffic:
- 20,000 to 35,000 AADT on the major roadway and
- 15,000 to 30,000 AADT on the minor roadway.
Skew Angle:
- Maximum of 30 degree.
Right of Way:
- A SPUI normally requires 30% less right-of-way than a diamond interchange.
Number of Lanes:
- 4 or more thru lanes based upon future capacity needs.
- Provide dual left turn lanes on the initial improvement since the addition of more lanes in the future is very difficult to construct.
- Provide a minimum 4 ft. lateral clearance between opposing left turn movements.
Turn Radius:
- 160 – 300 ft. for left turns
- 100 – 120 ft. for right turns.
234.4.1.19 Lighting, Signing, Pavement Marking and Signals
The placement and maintenance requirements of lighting, signs pavement marking, and signals must be considered early in the design process. Support poles within a SPUI will be placed outside the traveled way. Due to the long distances traveled in a turn movement, advance signing and secondary signal indications are necessary. Standard MoDOT equipment will be used.
Lighting:
- The most important design principles are uniformity of light and minimization of glare.
- SPUI mainlines and crossroads will be well lighted
- Good lighting is provided at the ramp junctions.
- The central intersection area is the most important area of the interchange and will be well lighted.
- The use of wall-pack lighting units along the vertical walls of
the bridge within the interchange is discouraged.
- Lighting is placed on signal poles.
- Detailed information is contained in EPG 901 Lighting.
Signing:
Since a SPUI is an uncommon and complex intersection, special attention to signing is necessary. Appropriate signing will be provided to avoid confusion. Important points to consider when providing signing are:
- Overhead guide signing is recommended for the approaching crossroad
- 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 should be placed at or just beyond the point where the left turn lane is fully developed.
- At least one set of “WRONG WAY” signs should be placed on the exit ramp to discourage wrong way traffic.
- Detailed information is contained in EPG 903 Highway Signing.
Pavement marking:
Since a SPUI is an uncommon and complex intersection, pavement marking must be given special attention. Appropriate pavement marking should be provided to avoid confusion. It is important to use white extension lines (2-foot long with a 4-foot gap) in the left turn lanes to provide guidance through the intersection area. However, since inclement weather and normal wear will reduce their effectiveness the use of durable markings is recommended. Detailed information is contained in EPG 620.2 Pavement and Curb Markings.
Signals:
A typical SPUI uses basic gap timing combined with vehicle detection in advance and at the stop bar. Interconnecting signals along the arterial route will improve coordination of through movements resulting in more efficient performance. Detailed information is contained in EPG 902 Signals.
A typical SPUI has three signal phases. One phase controls both crossroad left turn movements, one phase controls crossroad through movements, and one phase controls both off ramp left turn movements.
The change period (combination of yellow plus red timing) will need to be longer than normal for the left turn movements given the slow speeds and distances traveled.
234.4.2 Maintenance and Emergency Crossovers
Maintenance and emergency crossovers may be constructed on freeways and expressways so that maintenance, emergency and law enforcement vehicles can avoid extremely adverse distances. These 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.