The primary purpose of a superstreet is to improve traffic flow and safety along a corridor through the use of an economical and environmentally friendly solution.

• Improved Traffic Flow
• Increased Safety
• Economical Solution
• Environmentally Responsible

To say superstreets reduce travel time may seem a strange idea after seeing that vehicles attempting to turn left or go straight though an intersection from a side street are actually redirected to a U-turn roughly 600 to 1000 feet away and are then channeled back toward the intersection to complete their movement. However, everyone can appreciate the value of spending less time waiting at a traffic signal for the red light to turn green.

The superstreet concept was designed to decrease the time vehicles are stopped at the intersection thereby decreasing travel time. If the time a vehicle remains stopped is reduced, then traffic "flow," or progression, is also improved. A positive effect begins to domino along the roadway corridor because better flow translates into the ability to manage more vehicles with the existing roadway. In effect, it optimizes the use of an existing roadway system and reduces both delay and travel time through intersections.

The superstreet concept is based upon the relationship between the number of phases an intersection manages and the amount of available green time that allows traffic to flow. The fewer the number of phases, that is all traffic movements (lefts, throughs and rights) that are assigned "green time" simultaneously, the more green time that can be assigned to a given phase. The graph below compares the typically percent of green time at a signalized superstreet intersect and a signalized convential intersection. Due to the lower number of phases, additional green time can be assigned to the major roadway, which increases the capacity, traffic efficiency, while reducing delay motorists would experience.

The other major benefit to a superstreet is the improved traffic signal coordination, that is coordination of the green times between signals. Signals along a corridor are typically coordinated in the heavier direction of traffic. While this synchronization improves traffic flow in one travel direction, often times vehicles traveling in the opposite direction may only experience coordination for a few signals. A superstreet intersection however enables traffic signal coordination in both travel directions. Essentially by implementing a superstreet through an intersection or corridor, the roadway is transformed into two one-way streets. Think of traveling along a typical one-way street through a downtown area, whether it's McDowell Street in Raleigh or 6th Avenue in Manhattan. The signals are usually well coordinated so in theory once you make one green signal, you should make the next, and the next and so on, stopping at most only stop once at most (depending on other conflicts along the roadway).

• View a simulation of a typical signalized superstreet (8.1 MB) - Courtesy of the Maryland State Highway Administration.

Superstreets reduce the risk of crashes and specifically the risk of severe crashes such as side-collisions or T-bone type accidents. Since the superstreet concept eliminates the two movements that are statistically considered a higher risk for serious injury – side street throughs and lefts - the likelihood of severe and fatal incidents at the superstreet intersection are therefore reduced. For the same reasons, pedestrians also benefit from the safety features of superstreets. Due to the simplification of traffic flow and the reduction of potential conflicts with turning vehicles, pedestrians face fewer conflicts threats to their safety when crossing at superstreet intersections.

The illustration below (left) shows all the potential points where vehicles moving in conflicting directions may intersect, or crash at a convential four-way intersection . These “conflict points” are represented by the green, red and white circles that also indicate crash type. As depicted, there are 32 potential conflict points for the conventional intersection. Of these, 16 represent the opportunity for severe collisions. Collisions between vehicles traveling perpendicular to one another statistically result in the most severe injuries as drivers and/or passengers receive the brunt of impact from the side where they are the least protected.

Below (right) is the same illustration for the superstreet with all the potential conflict points identified. As depicted, there is a dramatic reduction in the number of conflict points - almost 50 percent. This decrease translates to a significant reduction in risk to motorists. The most important safety impact to note is that the conflict points for severe injury crash types are reduced from 18 for the conventional intersection to 2 for the superstreet.

Potential conflicts at a convential four-way intersection

Potential conflicts at a superstreet intersection

From an economic view point, the superstreet is an effective tool for reducing construction costs. In the past, when a conventional intersection had become ineffective and operated at unacceptable levels of delay, it was common practice to consider an interchange to relieve congestion. Fortunately, there is an additional choice which preserves the existing roadway, delays or potentially eliminates the need for an interchange and provides balanced access to both sides of the roadway.

Conversion from a divided highway to a superstreet configuration requires only minimal roadway construction and land procurement. The "footprint" of a typical superstreet intersection can be contained within an area of ten acres. The total expense to the public averages close to 2 million dollars with design and deployment complete within 12 months.

However, this is not the case for a conventional interchange which requires significant construction. The "footprint" for a typical interchange can impact as much as 90 acres with the total expense to the public reaching more than 7 million dollars to construct. Additionally, bridge construction associated with an interchange significantly lengthens the construction schedule and delivery can take anywhere from 18 months to 2 years.

The superstreet concept is an excellent example of environmentally sensitive design sometimes referred to as building “green.” Common goals for design using this philosophy apply across every discipline of engineering and include protection of natural resources and environment, optimizing existing facilities, recycling resources, spreading environmental impacts over long-range projects, creating communities versus sprawl and encouraging mix-use development and multi-modal transportation access.

The operation of superstreets plays an important role in reducing environmental impacts in several ways. Superstreets reduce the amount of time vehicles remain stopped and how this improves progression through entire corridors. The improved traffic signal efficiency results in less time vehicles spend idling. Therefore, fewer pollutants from exhaust are released into the atmosphere, air quality is improved and fuel consumption is also conserved.

There is also a significant difference in the physical footprint of superstreets as compared to interchange alternatives. Ten acres of land use impacts versus ninety acres of drainage design, bulldozing, service roads and access control create a vivid depiction of environmental advantages. Additionally, superstreets can delay or potentially eliminate the need for interchange construction while optimizing the capacity of the existing roadway.