Most of us will agree that suburban arterials—signalized roadways with four or more lanes—in the U.S. tend to operate very poorly these days. They are generally congested and experience far too many collisions, due in part to growing traffic demands that probably will not stop growing any time soon. Unfortunately, engineers tasked with fixing suburban arterials do not have many good solutions available.
Conventional traffic engineering solutions like actuated signals, turn bays and signal systems have generally been exhausted. Widening projects and bypasses are expensive, environmentally disruptive and may not help operations much. Flyovers and interchanges also are expensive and unpopular with roadside businesses left in the shadows. Intelligent transportation, transit, demand management and other possibilities have not yet proven helpful on suburban arterials.
Superstreets, part of a menu of unconventional arterial designs that this author has worked on for the past 15 years, are a promising solution for suburban arterials. They have the potential to move more vehicles efficiently and safely through the same arterial pavement as conventional arterials, at grade, with minimal disruptions to the surrounding environment and businesses. The purpose of this article is to familiarize you with this design and encourage you to include it among the alternatives whenever you are thinking about arterial improvements.
Super use
A superstreet works by redirecting left turn and through movements from side streets. Instead of allowing those to be made directly through a two-way median opening, as in conventional design, a superstreet sends those movements to a one-way median opening 500-800 ft downstream, as shown in Figure 1. Thus, a side-street through movement will be made by a right turn, then a U-turn, then another right turn.
The results from this redirection are dramatic. Traffic signals now require only two phases instead of the four or more phases—with green arrows for left turns from both streets—usually required at a busy two-way median opening. Since every signal phase introduces extra lost time for all motorists, this reduction in phases means significant time savings for everyone. In addition, the superstreet intersection shown in Figure 1 only has 14 conflict points—places where vehicle streams cross, merge or diverge—while a conventional intersection has 32. Since each conflict point adds another way for a vehicle to get hit, superstreets are likely to be safer.
The most profound change provided by a superstreet is in progression, which is the ability of vehicles to move along a road at a steady speed hitting one green signal after another. With a superstreet, the signals that control one direction of the arterial have nothing to do with the signals that control the other direction. This means that a superstreet will operate like a pair of one-way streets, and that perfect progression is possible at any speed with any signal spacing. This is an extraordinary capability; conventional arterials cannot approach this efficiency even with excruciating control of accesses and signal installations.
Superstreets are not brand new. Invented by Richard Kramer, a traffic engineer in Huntsville, Ala., in the early 1980s, superstreets have been in place on U.S. 301 on Maryland’s eastern shore for years. Michigan has over 1,000 miles of arterials with median U-turns which, as described below, is a closely related design. Many states make extensive use of one-way median crossovers that have some superstreet characteristics. No one designing a superstreet should feel like they are re-inventing the wheel.
Super strengths
Travel time savings top the list of superstreet advantages. Due to having only two signal phases and perfect progression, a typical six-lane superstreet should be able to accommodate a total of 75,000 vehicles or so per day at a decent peak-hour level of service of C or D.
For more specifics, this author and others have employed microscopic traffic simulation, particularly FHWA’s CORSIM program, to model travel times. In one of our efforts, we examined a 2.5-mile arterial in suburban Detroit that had five large, irregularly spaced signalized intersections. Keeping the amount of pavement and the signal-timing policies the same between alternatives, we showed that the superstreet would produce a 22% corridor-wide travel-time savings over a conventional design. In corridors with side streets that were not quite so large we would expect the savings to be greater.
Of course, related to travel-time savings are other important variables like emissions and fuel consumption. No reliable estimates exist at this time for these other variables, but the emissions and fuel-consumption savings for superstreets are likely to be large on typical suburban arterials.
Safety is impossible to simulate at this point in our profession, and full superstreet installations are limited in the U.S. thus far so to estimate the safety potential of this design we must turn to the literature on related designs.
Here, the news is encouraging: It appears as though superstreets will offer significant safety advantages over conventional arterials. Good evidence comes from Florida, where research showed that a right turn followed by a U-turn was much safer than a direct left turn out of a side street. In addition, our own recent research in North Carolina found very few collisions caused by U-turns on main streets with medians. The best evidence, though, is from Michigan: substantially lower collision rates on Michigan’s signalized arterials for median U-turns compared with conventional designs.
Related to safety, another advantage of superstreets is speed control. On a superstreet, agencies can set progression speeds as high or low as they wish (by location, direction, time, day of week or any number of ways) without changing the percentage of undelayed vehicles. Drivers will adjust quickly to the progression speed established, so as long as they obey traffic signals speeds will be controlled. Agencies can then reassign enforcement resources to duties other than speeding, enhancing safety and security elsewhere. In addition, a superstreet allows installation of any number of new traffic signals without changing the quality of progression, so engineers will no longer have to battle politicians and the public over signal installation.
The treatment of pedestrians is another superstreet advantage. Figure 1 shows the pedestrian crossing is completely signal controlled. At a superstreet, crossing pedestrians do not have to play “a game of chicken” with turning drivers like at a conventional intersection. In addition, while crossing a superstreet a pedestrian is looking directly at oncoming vehicles.
Super debate
A superstreet has some perceived disadvantages. However, these can all be mitigated. The largest of these is the presence of heavy side-street volumes. At some point side street left and through volumes become so heavy that their extra travel times outweigh the savings of other vehicles. In that case, designers should turn to the median U-turn design mentioned above and shown in Figure 2. A median U-turn redirects left turns but allows the side-street through movement to be made directly. This design has most of the advantages of the superstreet and provides tremendous travel-time savings over conventional design for most volume combinations even with heavy side-street through movements.
Another perceived disadvantage of the superstreet is the wide median needed to accommodate large vehicles making U-turns. Larger but still common trucks have turning radii of up to 45 ft, which could mean superstreet medians 42 to 66 ft wide to accommodate those vehicles within the travel lanes. However, designers should be aware of several ways to minimize that width, including:
- Not every median opening has to accommodate the largest design vehicle;
- Strengthened paved shoulders;
- Bulb-outs, as Figure 3 shows;
- For a median U-turn, putting the median and crossovers on the side street instead of the main street; and
- Again in the median U-turn realm, using roundabouts on the side street instead of median crossovers (a “bowtie” design) as Figure 4 shows.
Driver confusion is a concern with any new design. However, there are several reasons for believing that drivers will quickly adapt to superstreets.
First, similar designs like the median U-turn have been successful in parts of the U.S. for years. Second, in recent years drivers have adapted quickly to new designs like roundabouts and single-point interchanges. Third, good traffic control devices are available, such as the devices Michigan uses for its median u-turns.
Finally, like many designs with wider medians, superstreets may be perceived to harm roadside businesses, particularly businesses not at median openings that attract left-turn pass-by trips. However, these effects should be mitigated for superstreets in several ways, including that:
- The business-harm perception during project planning is typically worse than the reality after an improved arterial opens;
- There is typically no net communitywide negative effect;
- Travel-time savings mean more happier customers getting to more businesses quicker; and
- Perceptions of safety, slower speeds and better pedestrian access should all help.
Super potential
Superstreets offer a promising solution to the mess on most suburban arterials. They offer more efficient and safer travel, at grade, in an atmosphere of controlled speeds that welcomes pedestrians. The potential drawbacks, like heavy side-street through volumes, wider medians, driver confusion and lost business, can all be mitigated. Those looking at alternatives for suburban arterials owe it to the drivers and taxpayers to at least examine this promising alternative.
More information on superstreets and other unconventional designs is available from several sources. Good references are also available on the web: TME
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