A well-designed traffic management program can cost-effectively increase roadway throughput up to 25% while improving safety and reducing traffic volume on residential streets.
The primary methods used to achieve these benefits are traffic signal control, traffic monitoring, and access management. When combined with traffic-calming techniques on residential streets, a community's quality of life can be greatly improved. Today's techniques are driven largely by advances in technology. However, there are many “tried-and-true” methods used to make travel on local roads safer and more efficient.
Many U.S. communities have experienced steady or rapid growth—and hodgepodge development—over the years, creating major arterial highways lined by driveways, littered with traffic signals, and surrounded by retail outlets. Traffic from these congested highways eventually spills onto the residential streets of nearby neighborhoods. Residents react by insisting upon measures to stop the flood of “cut-through” traffic. Too often, the result is the introduction of barriers that simply shift the traffic elsewhere, ultimately making things worse.
To truly address the problem, officials should study the functionality of the entire network and identify root causes of congestion. Because capacity limitation is usually to blame and building new capacity is both difficult and cost-prohibitive, a comprehensive traffic management program is often the best approach.
CONTROL ACCESS TO CONTROL ACCIDENTS
The most traditional approach involves use of access control. Eliminating or combining driveways, constructing medians, and installing dedicated turn lanes can reduce and manage vehicle conflicts that lead to congestion. A Federal Highway Administration (FHWA) study found that access management improvements produce tremendous safety benefits, reducing crashes 40% to 70%.
While access management offers benefits, it can be difficult to implement. Property owners often have legal access to a public right of way, and any effort to restrict access is met with legal and political resistance. Eminent domain can be used to acquire a limited-access right of way line, but that can be a politically unpopular and expensive way to achieve better access control. There are more creative ways to approach the problem.
Make access management a requirement of new development. During development, require property owners to provide cross-access between parcels and, where possible, insist on installation of a service road network with as few access points to the main highway as possible. Discourage small, piecemeal developments that result in large numbers of driveways. A highly congested road carries fewer cars than one flowing smoothly, and people will avoid a business if the location is too difficult to reach. Also, keep access to major developments as far away from intersections as possible. Too many signals adjacent to a major intersection can dramatically reduce the intersection's throughput and hamper safety.
In established areas, look for opportunities to consolidate access points and remove underused signals. If an area is primed for redevelopment, work with property owners to reconfigure access during redevelopment. Try to consolidate entrance locations and place them in optimum locations. If possible, install service roads or turn lanes to improve traffic flow in front of the property.
PHASE THE SIGNALS
The phasing and timing of traffic control devices usually provides quick, cost-effective enhancement of the performance of a particular corridor. According to the FHWA, there are more than 330,000 traffic signals nationwide and up to 75% could benefit from more frequent phasing adjustments, whether manual or using real-time roadway sensors. Optimized phasing can have a benefit-to-cost ratio as high as 40:1, due to the low cost of retiming/optimizing signal timings. The FHWA has launched an aggressive program (www.ops.fhwa.dot.gov) to increase awareness and provide tools to state and local governments.
Older traffic signals use preset sequences determined through traffic studies. Because volume can change significantly over time, these signals can quickly become out of synch with actual conditions, leading to congestion. In fast-growing areas, timing should be updated once a year to ensure maximum traffic flow. In slow-growth areas a three-year optimizing schedule usually is adequate. Any changes to timing should be based on a thorough engineering analysis of current traffic conditions.
At busy intersections along arterial highways, a simple fixed-time signal can create unnecessary traffic disruptions and contribute to congestion. In these locations, a more advanced demand-actuated signal system is the best choice. This type uses sensors to detect vehicles and triggers phasing changes only when warranted. While demand-actuated signals improve flow at intersections, they can create congestion along busy urban corridors where random phasing patterns may disrupt traffic flow between closely spaced intersections. This can be addressed by using traffic-control software to coordinate timing of multiple signals, coupled with real-time communications between adjacent intersections, to maximize traffic flow along the entire corridor.
COMMAND AND CONTROL
In heavily congested corridors, simple coordination of signal timing may not be enough to maximize throughput. Instead, an adaptive control system (ACS) may be required. This type of system links a network of traffic signals with a central computer that uses traffic data to develop and implement the timing of each signal. As traffic conditions change, the computer makes real-time signal adjustments to maximize traffic flow across the entire network.
ACS can operate independently or be incorporated into a more comprehensive system such as one that would be housed in a traffic management center; these rely on a range of high-tech devices to monitor real-time traffic conditions and identify problems like accidents. Such information can then be furnished to drivers using a combination of dynamic message boards, highway advisory radio, and broadcast traffic reports so they can—armed with current data—alter routes to less congested corridors.
There is a wide assortment of technologies available for collecting real-time traffic data. Remote-control cameras with zoom capability are among the most popular devices but require human monitoring to be effective. Many other products can effectively monitor traffic volumes, vehicle movement, and highway speeds without human intervention. These devices use various technologies such as microwave, infrared, laser, and video. In areas with toll roads and widespread use of radio-frequency transponders, strategically placed transponder readers also are a popular way of monitoring traffic.
Each technology has strengths and weaknesses. Some are susceptible to changing weather conditions while others may be affected by atmospheric conditions or interference from other electronics. It is important to fully analyze the capabilities of each and evaluate operating conditions before selecting a technology.
Another critical consideration is the communication network. Linking a system frequently requires installation of a private communication network. Fiber-optic cables have been the preferred technology in the past decade, but the cost of installing new fiber cable in an urban environment has prodded the development of wireless technology. The use of wireless communication for traffic management is rapidly maturing and shows promise.
MAKE USE OF TECHNOLOGY
Traffic signals and other transportation management devices are classified as intelligent transportation system (ITS) devices. The FHWA has established a National ITS Architecture to promote uniform standards and interoperability. When implementing a new traffic management system, following these standards is essential. Traffic issues don't stop at the end of a street, the city limits, or the county line. Often, implementing an effective traffic management solution requires jurisdictional cooperation. The National ITS Architecture provides for communications across jurisdictional boundaries, sharing information, and creating interoperability solutions.
These management techniques pertain primarily to major arterials. Along with this effort, another aspect of traffic management is to determine how best to use traffic-calming techniques to manage traffic in residential neighborhoods. Roundabouts, speed bumps, rumble strips, and the narrowing of travel lanes are just a few of the methods that can be used to make neighborhood streets safer and less attractive to cut-through traffic.
When using traffic-calming measures it is important to keep a few facts in mind. First, traffic diverted from a neighborhood street has to go somewhere, and you must account for that. Second, traffic calming can be popular with residents of one neighborhood and extremely unpopular with residents of another. Solicit input from neighborhood groups and residents, and be prepared for a deluge of requests from other residents for traffic-calming measures on their streets.
Third, recognize that the Manual of Uniform Traffic Control Devices (MUTCD, http://mutcd.fhwa.dot.gov) provides guidance on residential traffic management. The MUTCD discourages certain methods for traffic calming. For example, stop signs are not speed-control devices. Using one in a place where it is clearly not needed for intersection control could create legal issues, and stop signs are not effective at reducing speeds or diverting traffic. And finally, some traffic-calming devices can slow down or prevent the response of certain emergency vehicles. Consult with local emergency services to ensure they are part of the residential traffic management process.
CONSIDER THE COSTS
Sophisticated traffic management systems can be costly, as can computerized systems, which require a higher level of maintenance expertise. Don't let the cost scare you. Effective management systems can be affordable, and there are always innovative ways to fund them. Many metropolitan planning organizations have funds set aside specifically for local traffic management projects. Participating in a jointly operated traffic management center or having a larger government agency or private contractor provide maintenance can reduce costs.
Another way to conserve costs is to use a phased implementation program. While initial planning should be comprehensive, there is nothing wrong with prioritizing individual projects and developing an extended implementation schedule, as with any large public works project. It's not unusual for government agencies to implement traffic management improvements one corridor at a time.
Ultimately, properly selected and installed traffic management devices can make significant, cost-effective improvements to the quality of life in the United States.
— John L. Hibbard, P.E., is senior ITS project manager, and James H. Evans is transportation planning program manager with PBS&J, Atlanta.
Coordinated signal phasing has multiple benefits
The use of phasing and timing traffic control devices can deliver quick, economical, and effective traffic management in a particular corridor, as shown by a U.S. DOT study. Source: U.S. DOT