Phase 1 Cross street traffic, both directions, all movements
Phase 2 Serve-Me-Next phase
Phase 3 Left turns off of artery
Phase 4 Through artery traffic
"Serve Me Next"
Last updated 7/28/04
ABSTRACT
"Serve-Me-Next" is used here to describe a method of reducing multiphase traffic signal delay to certain vehicles, such as streetcars or buses operating in reserved lanes. No actual pre-emption sequence need invoked but at the conclusion of the phase in effect, the priority vehicle is given the right of way. This may be accomplished using existing non-conflicting phases plus additional phases that are skipped except when a priority vehicle is present.
This is not a new concept. It is presented here to demonstrate that it can be implemented at many existing intersections using the existing signal controller with minimal configuration changes and no internal modification.
Serve-Me-Next is minimally intrusive to motorists at intersections equipped with fully actuated free operating signals.
BACKGROUND
Many major intersections are equipped with exclusive left turn lanes and multiphase signals. While we won't discuss terms like "quad left" or "fully actuated" in detail here, in simplistic terms, a typical signal installation is "four way" where in typical peak and near peak operation, the cycle length is 120 seconds or greater. Depending on when a vehicle arrives, it may be waiting nearly an entire cycle.
Since delays of this type make schedules irregular and greatly discourage use of mass transit, various traffic signal pre-emption systems have been developed and used to reduce delay to mass transit vehicles, especially streetcars (light rail vehicles). Usually there are undesirable side effects, notably the omission of one general traffic phase while other phases are served two or three times, with resultant undesirable queuing and backups.
Transit vehicle priority has been implemented in a variety of ways. To one extreme, a railroad crossing protocol is used, with stopping of road traffic in advance of a streetcar's arrival and where the streetcar proceeds at near full speed. To the other extreme is no special or unusual treatment at all.
At least one instance of problems caused by the railroad crossing protocol has occurred, in Minneapolis, Minn. A newly opened (2004) streetcar line has resulted in traffic tieups because such a large percentage of the signal split had been taken by the streetcars and by signal clearance intervals. The other extreme, no priority, is exemplified in Baltimore, MD where it is not unusual for streetcars to take 15 minutes to cover a mile on Howard St. even during non-rush hours.
Serve-Me-Next is an intermediate, or less aggressive, approach to transit vehicle priority. It provides a better balance between maximizing intersection efficiency (throughput) and minimizing delay to transit vehicles.
Experience
No field research has been done and no data has been collected. We do believe that the benefits are intuitive and obvious. We do not know of any existing operating examples.
Suggested Usage
Serve-Me-Next is quite suitable for intersections operating at or near capacity where it is intended to use some form of transit priority anyway.
Serve-Me-Next is a worthwhile alternative for jurisdictions not favoring pre-emption using railroad crossing protocol.
A high quality of transit service should be attainable where most signalized intersections use railroad crossing protocol or predictive pre-emption and one to two signals per mile operate using Serve-Me-Next.
Operational Characteristics
To the motorist, the Serve-Me-Next phase appears to be called on demand in a manner no different from any actuated phase. The difference is that a Serve-Me-Next phase may occur anywhere in the signal cycle. There should be no safety issues so long as motorists observe and obey the signal indications, especially waiting for a green indication before proceeding.
Most of the time the streetcar will have to come to a complete stop. In most cases, even if there are several streetcars coupled into a train, the time it takes the streetcar(s) to start up and clear the intersection is less compared with the time traffic is stopped using a railroad crossing protocol.
Serve-Me-Next has less impact on at-capacity intersections also because in terms of pre-emption the signal controller "resumes where it left off". Most standard pre-emption sequences have the controller go to a specific phase following pre-emption. Here, compromise split calculations are necessary to try to achieve a desired split given the number of streetcars per hour and even this does not preclude more cycle failures on some approaches compared with others.
Design Goal
Delay to a priority vehicle should not exceed half the cycle length or 45 seconds whichever is less.
BASIC SERVE-ME-NEXT
In many instances, Serve-Me-Next can be implemented by re-wiring the external connections (backplane) and re-programming an existing controller using just the standard features described in its instruction manual. The key feature needed is overlap capability. There are some tradeoffs, for example there might not be enough circuits (phase modules; overlap cards) and a right turn once given a green arrow now has to rely on right turn on red instead.
Since there are several Serve-Me-Next phases in the cycle, presence detection (non-locking calls) must be used. Any detector malfunction would need prompt attention as the priority phases will produce unacceptable impact on traffic if put on recall.
Sample Implementations
A simple example of Serve-Me-Next is shown below. Not shown is green extension or red truncation for phase 4 with the concurrent vehicular movement but these features can also be implemented if desired.
("M" stands for "metrorail". Assume that transit operates in median of artery)
Phase 1 Cross street traffic, both directions, all movements
Phase 2 Serve-Me-Next phase
Phase 3 Left turns off of artery
Phase 4 Through artery traffic
Overlap card for transit uses phases 2 and 4
This more complex example shows how Serve-Me-Next can be implemented for transit lines on two intersecting streets. This plan, too, can be implemented on many older controllers such as a TCT (Crouse Hinds) DMK2.
Phase 1 East to North and West to South left turns
Phase 2 Serve-Me-Next for North & South transit
Phase 3 East & West through traffic and transit
Phase 4 Serve-Me-Next for North & South transit
Phase 5 North to West and East to South left turns
Phase 6 Serve-Me-Next for East & West transit
Phase 7 North & South through traffic and transit
Phase 8 Serve-Me-Next for East & West transit
Two overlap cards are used for transit in this example, one uses phases 2, 4, and 7 for north-south transit and the other uses phases 3, 6, and 8 for east-west transit.
Wide Area Detection
Serve-Me-Next may use advance detectors so a call is registered for the Serve-Me-Next phase several seconds before a streetcar arrives at the intersection. If the intent is to allow a streetcar to proceed without stopping some of the time, intersection efficiency is decreased if the Serve-Me-Next phase begins in advance of the streetcar's arrival. Intersection efficiency can also be decreased if there is a near side passenger stop and detection requires manual activation by the streetcar driver who might perform such activation too far in advance.
Single Ring vs. Dual Ring
Serve-Me-Next is less effective in a normal dual ring phasing since there is no way to assure that two concurrently timing movements terminate together. Therefore Serve-Me-Next phases can be placed in fewer locations within a dual ring signal cycle.
In addition, Serve-Me-Next may preclude dual ring operations because the controller does not have enough circuits (phase modules, etc.) to also provide dual ring operation.
The most common consequence of changing to single ring operation is that some slack time is given to a left turn phase as opposed to a through phase.
Lead Lag Phasing
Serve-Me-Next is less effective with lead lag phasing where concurrent through movements with or without permissive left turns occur in the middle of the lead lag sequence. Here the only way to place a Serve-Me-Next phase within what may be a lengthy lead lag sequence is to allow interrupting and resuming of one of the through movements. A split phasing consisting of a single ring lead lag without concurrent through movements in the middle may improve performance in intersections where streetcars turn, but may hinder performance in other intersections where streetcar movements cannot overlap any of the general traffic movements.
Serve-Me-Next Phase Length
Since streetcars may be coupled into trains, it is desirable to provide a variable phase length using passage time (unit extension) actuation by the streetcar(s). Depending on local acceptability, the Serve-Me-Next phase may terminate before a train of streetcars has completely cleared the intersection. On the other hand, a Serve-Me-Next phase may be made slightly longer than needed to allow a non-conflicting vehicular movement to occur concurrently using an overlap without the display of an unnaturally short green interval for that movement. Normally the Serve-Me-Next phases are not actuated or extended by general vehicular traffic nor are they part of detector switching.
Clearance Intervals
Unlike pre-emption sequences, Serve-Me-Next uses controller phases that can gap out or max out. Particularly where the intersection operates at or over capacity and it is desired to maximize efficiency, the clearance interval for the Serve-Me-Next phase must be dealt with carefully. It is important that another approaching streetcar or streetcar train not granted the right of way be able to stop before a proceed indication is given to conflicting vehicular traffic. Many accidents have occurred at railroad crossings where a motorist crossing the tracks after a train has passed collides with a second train on a different track. An effective solution is to have a signal head that displays the proceed indication for a streetcar for just a few seconds even though the Serve-Me-Next phase times out the passage of a streetcar train coming the other way. This may permit tight timing where vehicles can cross the track safely just as the end of a train passes.
Coordinated vs. Free Operation
Coordinated operation is not recommended together with simple Serve-Me-Next implementation unless the priority movement does not conflict with, and overlaps, the coordinated phase. Better results are had with older controllers where the split is enforced using the green maxes and the coordination is enforced using the force-offs.
If the vehicular phases are quite long to begin with, the Serve-Me-Next phases can operate as deductible phases and enough time will remain for the succeeding vehicular phase without the display of an unnaturally short green interval (disregarding capacity). The significance of using the green max to enforce the split here is that the force-off for the phase immediately preceding the coordinated phase may be placed after the desired start of the coordinated phase and be invoked only if a Serve-Me-Next phase was called earlier in the cycle. (The coordinated phase here would have to be sufficiently long to withstand a call to another Serve-Me-Next phase after the late termination of the preceding vehicular phase.)
ADVANCED SERVE-ME-NEXT
The following features require that a controller be specifically programmed for them. These features may not become generally available until manufacturers decide to incorporate the concept of Serve-Me-Next in controller design. In a few cases this can be done on-site where the technician is familiar with the micro-code of the controller.
Phase Truncation
Serve-Me-Next may be enhanced with a future feature that provides for the truncation of the conflicting phase currently timing including a coordinated phase. Criteria used may include the length of time the conflicting phase has been active, or a percentage offset in a coordinated background cycle.
Rest-At-Max
If the phase currently timing has been active for a reasonable length of time (as defined by and programmed by the jurisdiction and which may be the existing green max) this future feature will allow an arriving streetcar will pre-empt that phase immediately. This is contrary to the normal definition of full actuation where the max timer remains at zero prior to receipt of a conflicting demand and the phase currently timing with ongoing actuation retains the right of way until it gaps out or maxes out. In a Serve-Me-Next application a second "max" timer may be implemented to time a site specific "reasonable length of green" which may be less than the green max for a phase. However under no conditions would a phase be shorter than one ped interval plus one ped clearance interval if a pedestrian demand was being served.
Force To Single Ring
A controller may be configured for dual ring operation but when a Serve-Me-Next demand is received, this future feature will cause all concurrently timing phases to be terminated together without advancing to the compatibility barrier. This may involve allowing a gapped out phase to continue until the other phase gaps out or maxes out, or truncation of the phase that has not yet gapped out, or truncation of both phases, depending on the circumstances and site specific programming.
Accumulating Streetcars
The controller may limit the serving of Serve-Me-Next phases using such criteria as (1) whether streetcars are running behind schedule, (2) number of people on board the streetcar(s), (3) streetcars in both directions of travel waiting, or (4) a minimum time interval between Serve-Me-Next invocations to name a few situations.
Predictive Transit Vehicle Pre-emption
Serve-Me-Next does not preclude the use of methods that attempt to determine when a streetcar will arrive at an intersection and attempt to pre-empt the signal sequence accordingly. Serve-Me-Next can also improve the performance of predictive pre-emption in that just a small amount of stretching or squeezing of intervening vehicular phases may suffice. Except that the cost of implementing both systems may be undesirably high. Even such predictive systems are not perfect, there will still be instances when traffic is stopped well in advance of the streetcar's arrival, and/or instances where the streetcar has to stop and wait. By positioning the streetcar detector(s) the tradeoff between streetcar delay and intersection efficiency loss can be adjusted.
Cycle Proportioning
In coordinated operation, if a Serve-Me-Next phase is called, this future feature causes the time remaining for uncoordinated phases to be divided among the latter using a formula, possibly computed ad-hoc for that cycle, rather than by using subsequent actuations, gap-outs, force-offs, or omits. In addition, a late start to the coordinated phase may be allowed to occur in a cycle where a Serve-Me-Next phase was called.
Railroad Signals
With Serve-Me-Next, the streetcar driver must be attentive to both railway signal protocol and road traffic signal protocol. In railroad signal protocol, signals at intersections (turnouts, junctions, crossings) with conflicting traffic normally display the stop indication until a train desiring passage approaches.
BUS RAPID TRANSIT
Serve-Me-Next is suitable for BRT applications also.
Serve-Me-Next is less effective where buses (or streetcars) operate in mixed traffic lanes since the length of the Serve-Me-Next phases must be longer to allow queued vehicles ahead of the bus to get out of the way.
Serve-Me-Next is not intended to be invoked on every cycle as might occur if there are several buses per minute. Even if some general vehicular movement may occur concurrently during the transit phase, there is a significant loss of efficiency.
CONCLUSION
We believe that Serve-Me-Next as described here is a worthwhile tool to managing traffic flow at saturated intersections where mass transit vehicles are involved.
All parts (c) copyright 2004, Allan W. Jayne, Jr. unless otherwise noted or other origin stated.
The author is a member of the Institute of Transportation Engineers.
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