Superstreets

Introduction

Target Market

Arterials with Heavy Delays at an Intersection
Signalized intersections commonly cause major delays and congestion on roadways due to the time required for each phase. This design can reduce delays by having several phases running at the same time while minor-street through traffic is redirected.

Arterial Intersections with Large Left-Turn Volumes
Left turns cause major traffic delays. They interfere with opposing traffic and can require their own signal at certain intersections. Congestion can be reduced by removing left-turn movements from the main intersection and placing them at a location away from that area.

How Will This Help?

• Increases safety by reducing the number of conflicts at the main intersection. Not allowing left turns at the main intersection eliminates problem areas from the intersection and reduces dealing with opposing vehicles.
• Reduces delay, improves traffic flow, and simplifies signal timing at the main intersection. Removing left turns and eliminating multiple signal phases simplify and improve the intersection.
• Works well with existing access management plans. Superstreets allow easy access to commercial areas and limit the traffic effects on local businesses and industry.

Implementation Examples

 San Antonio, Texas
The San Antonio area has two locations with superstreets. One is on US 281 North (four intersections), and one is on Loop 1604 West (two intersections). Each superstreet provides a transition between the freeway and non-freeway sections of the roads. Both are considered short-term improvements until planning is completed and funding is acquired for more substantial upgrades.

A recent before-after study for Loop 1604 between SH 151 and Braun Road found travel time reductions of 14 percent during the northbound morning rush hour and 35 percent during the southbound evening rush hour. A before-after study on US 281 in San Antonio found that travel times were shorter and average speed higher, even though traffic volumes increased on the roadway, as shown in the table below.

North Carolina
North Carolina uses the superstreet concept onseveral roadways. Results of a 2010 study showed that superstreet intersections worked better than normal intersections at each location studied. Superstreets reduced the overall average travel time per vehicle traveling through the intersection.¹

Application Principles and Techniques

The superstreet concept typically applies to the intersection of a major road with higher volumes and a minor street with lower volumes. The overall right-of-way needs and design of the superstreet depend on the types of vehicles using the roadway. The superstreet should be able to handle large trucks, but the following methods can minimize the requirements:²

• Construct some lanes for passenger cars only. Ensure adequate signs that ban trucks from using these lanes.
• Allow larger vehicles to use the shoulder when turning. This may require a stronger pavement.
• Create larger turning areas at the intersection.
• Widen the median at the turn areas, but narrow it after the turn position.

Delays and traffic lines may occur if the design is not adequate. This effect should be considered when applying the superstreet concept. Providing enough space between median turn points can decrease the likelihood of these events. The larger spacing will also provide more flexibility for drivers trying to find the lanes needed for their direction.

The main intersection of the superstreet typically has traffic signals, but the U-turn may not. Standard rules for applying traffic signs and signals determine which traffic control devices to use at that location. The design requires a large number of signs to ensure driver understanding. Signs should be placed in areas to provide ample warning and direction so that the intersection functions well and no congestion is caused by improper use of the intersection.

The superstreet intersection should allow for pedestrians if they are expected to use it regularly. Changes include eliminating right-turn lanes or traffic islands to shorten the crossing distances.

Issues

Superstreets can cause driver confusion. Drivers may ignore the prohibited left turns at the main intersection or reject the concept altogether. Responsible agencies should educate the public to get them used to the idea of superstreets when the idea is first introduced to the area.

Other issues that need to be considered are as follows:

• The additional right of way and paved surfaces required to construct the modified U-turn lanes could have a major influence on cost.
• Sufficient signing for users is important to operational success.
• The operation and maintenance of superstreets can be costlier than other street designs.
• Left-turning vehicles can experience larger delays and distances due to the displaced site.

Who Is Responsible?

The overall responsibility of designing and executing a superstreet depends on the location. Superstreets typically involve high-volume highways and other arterials and are usually the responsibility of the local Texas Department of Transportation (TxDOT) office. The TxDOT office should work with local city officials because superstreets can affect local businesses and streets. City and state officials should have a close relationship to ensure adequate design and implementation.

Project Time Frame

The time required to create a superstreet depends on the current intersection and the available right of way in the area. Constructing the superstreet can take longer if the additional land is not readily available and must be acquired from outside sources. A typical superstreet can take several months or even years to create depending on available right of way.

Cost

The cost of a superstreet depends on the available right of way and the current state of the intersection. The cost will rise if the necessary right of way is not available and must be purchased from other owners. The overall cost may be higher than that of other intersection methods due to the possible signals and signs required for the U-turns. Practitioners should consider comparing the cost of a superstreet design to similar intersection designs, such as median U-turn intersections, before deciding to create a superstreet. The cost to build the superstreets in San Antonio was nearly $5.2 million for US 281 and $5.7 million for Loop 1604.³

Data Needs

Data that can help determine whether to use a superstreet include:

• Traffic counts for all approaches.
• Left-turn counts at the intersection.
• The current signal timing plan.
• The delay caused by the intersection.
• Speeds traveling through the intersection.
• Crash data.
• Available right of way.
• Current roadway lane, median, and shoulder widths.

Superstreets Best Practices

• Type of location: High-volume major streets, particularly at intersections with substantial left-turn volumes.
• Agency practices: Coordination between planning, design, safety, and operations.
• Frequency of reanalysis: After substantial land use changes or development, as travel increases or trips change in the area, and at the time of roadway widening or reconstruction.
• Supporting policies or actions needed: Capability to fund improvements, multi-agency agreements and policies where roadways cross jurisdictional boundaries, and driver education campaigns.
• Complementary strategies: Intersection improvements, pedestrian treatments, innovative intersection design, median U-turn intersection, and access management.

For More Information

Alamo Regional Mobility Authority. http://www.alamorma.org/index.cfm/projects/us-281-super-street/.

References

1. W. Hughes, R. Jagannathan, D. Sengupta, and J. Hummer. Alternative Intersections/Interchanges: Informational Report (AIIR). Report FHWA-HRT-09-060, Research, Development, and Technology Turner-Fairbank Highway Research Center, McLean, Virginia, April 2010.
2. J. E. Hummer, R. L. Haley, S. E. Ott, R. S. Foyle, and C. M. Cunningham. Superstreet Benefits and Capacities. Report FHWA/NC/2009-06, North Carolina Department of Transportation, Raleigh, North Carolina, December 2010. http://ntl.bts.gov/lib/37000/37800/37839/2009-06finalreport.pdf.
3. The Texas Highway Man. Superstreets. May 4, 2014. http://www.texashighwayman.com/superstreets.shtml.