Light Rail Preemption of Traffic Signals: a Question of Balance
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Light Rail Preemption of Traffic Signals: A Question of Balance Woodrow Hood III, Thomas Hicks, and Linda I. Singer, Maryland State Highway Administration As Maryland expands its network of light rail transit fa• emption; and reducing the disruption to the signal system cilities, an increasing number of situations arise where light caused by frequent light rail preemption. The field testing rail and traffic-signalized intersections must operate in con• and design of these strategies are addressed. junction with each other. These include at-grade crossings close to signalized intersections and light rail facilities that closely parallel arterial roadways where there are closely ^ I 1 he subject of this paper is balance in terms of spaced traffic signals on the arterial. These locations pre• I traffic numbers, that is, vehicles and people in sent the traffic engineer with unique problems that must JL quantity and travel time, and in terms of mind• be resolved to facilitate movement of the light rail vehicle set, that is, perception of needs and advantages. and other vehicular traffic through the area. In most cases, On the one hand, there is a continually increasing the periodic preemption of a traffic signal at an isolated number of vehicles on local roadways; on the other, intersection will cause only momentary disruption to ve• there is a move to get people out of their cars and on hicular traffic. It is when more frequent preemption occurs, to buses and light rail service. The emphasis on mass or when the signal operates as part of a coordinated traffic transit is particularly important in the era of the Clean signal system, that problems arise. These include disrup• Air Act Amendments (CAAA) and penalties for not re• tion of traffic signal coordination on the arterial, excessive ducing vehicle emissions in nonattainment areas. clearance intervals needed to ensure that right-of-way has Our customers tell us that, assuming there is not a been cleared for the light rail, and excessive delay to ve• vast disparity in the cost of car versus transit, people hicles waiting at the preempted traffic signal. In addition make their decisions on which mode of transportation no two locations have the same characteristics or solutions. to use on the basis of time and convenience. The more The traffic engineer must consider these and other factors their travel time can be reduced, the more likely it is when developing a traffic control scheme for each individ• that travelers will be enticed to use buses and light rail, ual location. Maryland is evaluating several traffic control leading to less congestion and pollution. However, there strategies for these types of locations. They include allow• will still be people in their personal vehicles for local ing the signal to cycle through other nonconflicting phases transportation and for getting to destinations not served while the light rail vehicle passes through an intersection; by mass transit. allowing the signal controller to select, on the basis of traf• The result of greater transit usage is more and more fic demand, which phase should be serviced first after pre• situations in which transit facilities come into conflict 285 286 SEVENTH NATIONAL CONFERENCE ON LIGHT RAIL TRANSIT with roadway facilities. Simply put, how do traffic en• compound the problems for our traffic engineers, no gineers fairly balance the needs of car and transit users two situations are identical. Each presents new prob• when they both must approach and traverse a heavily lems and requires new solutions for configuration of the congested intersection? roadways and the coordination necessary for the Whatever the resolution in a particular situation, signals. there must be a balance between the legitimate needs of The problem—and the challenge—came down to drivers and transit passengers. In achieving that bal• finding a way of freezing only the movement at a par• ance, a measure of priority must be provided to the ticular intersection that would conflict with the transit transit operation if mass transit is to be made appealing vehicle without immobifizing the entire intersection and enough to get drivers out of their cars and onto mass signal system. The trick was to keep nonconflicting transit. vehicles moving by efficiently using the available green Most often, that priority has been provided by traffic time in phases that did not affect the transit vehicle. signal preemption. Such preemption in the cases of light Indeed, this is a delicate balance. rail and freight train service halted some or all traffic flow at the intersection until the transit vehicle or freight train could move into and through the intersection. In some cases the complete halting of all traffic flow at the MEETING THE CHALLENGE intersection could not be avoided owing to the way the intersection had been laid out. In others, where the geo• The answer to this challenge lies in newly available tech• nology and a commitment by policy makers to customer- metries are conducive, a balance has been achieved by driven quality in transportation and to compliance with taking advantage of state-of-the-art technology. new federal requirements under the CAAA and the In- termodal Surface Transportation Efficiency Act. THE PROBLEM Maryland has been exploring several strategies to ad• dress the problem. These include Maryland is currently constructing extensions to its light rail system in the Baltimore downtown and met• • Allowing the signal to cycle through other noncon• ropolitan areas. For right-of-way concerns and the ac• flicting phases while the light rail vehicle is passing cessibility of most of its customers, the light rail lines through the intersection; are located on or immediately parallel to major arterial • Allowing the signal controller to select, on the ba• roadways. In the more urbanized areas, they also cross sis of traffic demand, which phase should be serviced closely spaced, signalized intersecting streets. Most are first after the preemption; at-grade crossings. • Using different track clearance sequences—the sig• When traffic signals on these arterials and intersect• nal sequences used to clear motorists from the tracks; ing streets are very close together, they are coordinated • Finding ways to reduce disruption to the coordi• for efficient vehicle movement. When traffic engineers nated signal system that is caused by frequent light rail preempt a signal within a traffic signal system, a mea• preemption; and sure of coordination is lost on the arterial roadway, as • Holding the transit vehicle until the best possible well as on the cross roads. Preemption sequences for moment within the traffic signal cycle to allow the most transit vehicles are extremely disruptive to vehicular efficient movement through the intersection. traffic because the priority is given without considering the impacts on the arterial or the entire system. As one These different strategies are now possible through might expect, the result was worst at closely spaced the use of state-of-the-art traffic signal controllers. The traffic signals and at high-volume intersections. Vehicles emphasis in Maryland has been on the use of signal get stacked up between the signals and cannot move. systems to provide smooth traffic flow along arterial Excessive clearance intervals are needed to ensure that roadways. By definition, signal preemption runs con• the right-of-way has been cleared for the transit vehicle. trary to this approach. But the new controllers allow Long queues develop at the intersecting roadways. Fi• minimal disruption, thereby adhering more closely to nally, there are excessive delays to vehicles waiting at our original approach than was possible with earlier the preempted traffic signals until the signals get back controllers. The previous technology was limited in its in step with one another. ability to balance the needs of motorists and transit We realized, as our traffic engineers were asked to users because of limitations in the "thinking" ability of help our transit sister agency with signal coordination, the software. In this case, the policy emphasis drove the that these circumstances were unacceptable to our cus• development of more advanced and specific-results- tomers, both transit users and motor vehicle drivers. To oriented software. HOOD ET AL. 287 LEARNING GROUND indication, thus ensuring that the bus got though the intersection without having to stop for a red signal. This In August 1993 Maryland successfully implemented its would prevent a bus from having to stop twice at the first experimental bus preemption system. It was in• same intersection, once for the red signal and again on stalled along MD 2 (Ritchie Highway) between Balti• the far side of the intersection to pick up and discharge more and Annapolis, an arterial that carries average passengers. daily traffic (ADT) of 32,000 to 35,000. This phase extension was provided by giving the transit vehicle an additional extendable green interval What We Did to proceed through the intersection. The additional green time would be borrowed from the minor move• The preemption system consisted of bus priority control ments and then returned to the next signal cycle. The of 13 signalized intersections that operated as a coor• additional green interval was up to 15 to 20 sec. dinated signal system. The system allowed express buses The phase reservice (Figure 3) was used at a location to use any of three types of priority controls, depending where the bus made a left turn off the arterial to service on whether the bus stop was located on the near side a major park-and-ride lot. This maneuver had the effect or the far side of the intersection (in the Ritchie High• of serving the left-turn phase twice (if necessary) within way case, we did not relocate any of the bus stops). The the same signal cycle.