Traffic-signal coordination is a tool traffic engineers have applied for years to increase throughput and reduce delays. A system with good signal progression also benefits from reduced accidents and reduced emissions. The principle is simple: maximize the vehicles moving and minimize stops and delays. This is accomplished by keeping vehicles in platoons that progress down the street arriving at a green light at each downstream intersection. Sounds easy enough. But how many traffic engineers have heard the following questions at a social gathering as soon as people hear your profession:
“So why isn’t every street coordinated?”
“How come I have to wait at a red light when no one’s around?”
“Can’t you just make them all green?”
Traffic-signal coordination is a process that started decades ago. With computer software and various detector technologies, the ability to more effectively coordinate a system is becoming more efficient. For the last 20 years, adaptive-signal control systems have allowed real-time coordination based on real-time vehicle detection and on-line optimization. So why does the transportation community continue to struggle to maintain and implement optimal signal timing? The difficulty in traffic-signal timing is often a manpower and maintenance issue for the local agencies. With insufficient resources to manage a signal system, updates are often much lower on the priority list. This article identifies the benefits that updated traffic-signal timing can provide to the system travelers.
Fiber provides healthy return
A recent advanced transportation management system (ATMS) evaluation identified the benefits that Utah’s Salt Lake Valley system is providing. The study quantified the value that each individual ATMS component provides based on delay, safety and emissions. These individual components included: ramp meter, CCTV, variable message signs (VMS), incident management teams (IMT) and traffic-signal coordination. In the Salt Lake Valley, there are over 900 traffic signals. The system communicates via fiber optics to more than 600 traffic signals, communicating through the three regional transportation operation centers (TOCs). The fiber system cost over $51 million to install, almost half the entire ATMS system costs. The estimated annual ATMS benefit is $179 million. The traffic-signal coordination effort accounts for 87% of the system-wide social benefit.
To model or not to model
Ideally, before and after travel time runs are provided for each coordinated timing plan. This would include not only the benefits to the main corridor, but the possible negative effects to the side-street delay. A statistical sampling of these runs allows definitive statements about the signal-timing benefits. However, the costs for collecting this data are often burdensome. That is why many agencies rely on computer models to justify the improvements from coordination. Because of a specific request not to rely on modeling, the signal-timing benefits in the Salt Lake Valley are based on sample corridors where travel-time runs (before and after) are collected and then extrapolation from the findings to estimate the system-wide signal coordination benefits.
Pain-free installation
Signal coordination can be accomplished without a communication network. The time required to implement, update and maintain makes it more difficult for large systems unless sufficient manpower is allocated to the signal system. Many agencies found that maintaining a coordinated system of hundreds of signals is too onerous a task until a centralized communication system is installed.
Once the communications are in place, TOC operators can more readily update and modify signal timing to efficiently manage the traffic signals. The Salt Lake ATMS operates more than 50 independent coordinated corridors throughout the valley. Communication between the signals and the TOC allows more frequent updates to corridor coordination. General coordination can be done in the field by hand if sufficient resources are allocated, but some benefits are only available when a real-time communication system is in place. These benefits come from incident response timing changes due to special events, accidents that cause re-routing and inclement weather. In 2003 there were more than 600 unplanned signal-timing changes that resulted in an estimated $6.4 million in benefits due to reduced commuter delay. The signal-timing benefits for special events and inclement weather are based on the number of events in a year as shown in Figure 1. Table 2 shows the estimated annual additional benefit to drivers due to these dynamic changes that are only available because of the communication.
It’s empirical
The lack of available travel-time data is a primary problem encountered when evaluating the benefits of signal coordination. It is more difficult, more expensive and more time consuming than using simulation models. Computer time is more cost-effective and timely. For this evaluation, empirical travel time runs are collected before and after coordination from nine of the 50 coordinated corridors, or 18% of the system. The actual benefits from the sample corridors are then extrapolated to the remainder of the system. The coordinated system includes 136 miles of roadway with 434 coordinated signals.
Travel-time data for all nine corridors is collected for five time periods: morning, mid-morning, noon, afternoon and night. Other data used to estimate the benefits of signal coordination includes corridor length, the number of signals and spacing and the annual average daily traffic (AADT).
When using empirical data, often only the main street is factored. The main street should show improvements because that is being coordinated. Often the increase in delay to the side streets is forgotten in the evaluation. Then everything is a benefit and nothing is a disadvantage. Main street travel-time benefits often come with an expense to the side streets. The negative effect on side streets, however, is usually not large enough to outweigh the overall benefits to the system and most findings are that a negative 15-25% side-street delay increase should be factored into the mainline corridor benefits. This implies that for a typical travel-time evaluation, 75-85% of the measured mainline benefits are actually received when the cross streets are considered.
Once the information from the existing travel-time runs is processed, the average benefit per roadway based on signal spacing, AADT and geometry is identified. The benefits are extrapolated to the remainder of the coordinated corridors based on similar traffic characteristics.
Day to day
Signal coordination is typically the most beneficial component of an ATMS because the coordination provides benefits for the entire day, every day of the year. The other ATMS components are typically beneficial during incidents only. Consider an average intersection where two five-lane facilities cross. If both roads carry an AADT of 30,000, then 60,000 vehicles will pass through the intersection. If each vehicle saves just one second in delay, then more than 6,083 vehicle hours of savings would occur throughout the year. Using an average occupancy of 1.27 and $11.84 per hour time value (UDOT time value), that one second per vehicle savings equates to $91,473 of user-delay savings for that single intersection. It is easy to see how a system-wide savings, where 10-20 seconds of savings per vehicle per intersection are realized, could quickly add up.
Utah undertaking
Traffic engineers often focus efforts on the freeways and interstates that are high-speed and high-volume roads. On these facilities, ATMS devices, such as VMSs and CCTVs, are primarily to react to accidents, where the most people are inconvenienced. However, in urban areas there are many more surface roadways with signals that affect the drivers all day, every day. It is an arduous task to install, update and maintain a signal system for a metropolitan area. Sufficient resources need to be applied to allow these systems to function effectively. With the signal system providing 87% of the benefits to the Salt Lake Valley ATMS, it’s clear that this aspect of ATMS provides the biggest return on investment and in one year justifies the installation costs for the communication system.
Whenever possible, statistical validation of travel times, side-street delays, arrivals on green and a number of other factors should provide the empirical evaluation as to the effectiveness of a signal system. Using sample corridors and extrapolating the findings to other similar corridors allows the system benefits to be estimated. This process is now being applied to other areas of Utah to develop a priority list of corridors to connect via fiber and then to coordinate since insufficient resources exist to provide communication and coordination to the entire state’s signals.
Besides traditional coordination and updates, the ability to dynamically change coordination plans due to inclement weather, incidents or construction has provided substantial benefits, so much so that surface-street cameras are becoming a priority at critical intersections so TOC operators can update the timings based on field observations along principal corridors.
TME
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