Tuesday, April 24, 2012

Snappy Signal Control, Brookline Street @ Granite, Waverly, Cambridge

By: Kevin McCarthy

Too often traffic signals are not set in the most efficient manner. When designing a signal cycle, the primary goal should be to minimize delay for all users including vehicles, bicycles, and pedestrians. The easiest way to minimize delay is to make cycle lengths as short as possible while still providing adequate time for queues to disperse and for pedestrians to safely cross the street. One way to achieve the shortest possible cycle length is to have an actuated signal. An actuated signal uses detectors to determine when phases start and end. This ensures that as little green time is wasted as possible. When an actuated signal has short critical gaps the intersection can cycle faster providing less delay for all. When a signal is timed in this way it is referred to as snappy. An example of a snappy signal control is at the intersection of Brookline St., Granite Ave., and Waverly St. in Cambridge, MA

Intersection of Brookline St., Granite Ave., and Waverly St. in Cambridge, MA 

Signal Timing Plan

The analysis of this intersection was conducted on Friday April 13th, 2012 at 5pm to observe the snappy controls and collect data. The following is a ring diagram showing a typical cycle for this intersection. This is the cycle that is used for delay analysis.

Ring Diagram Showing a Typical Signal Timing Plan for the Intersection

EBL Vehicle
The above ring diagram is only a typical timing plan not a set timing plan because the intersection is actuated, not pre-timed. An actuated signal has detectors which control the start and end of each phase. This means delay can be reduced even further by allowing certain phases to be skipped when there are no cars, to end when a critical gap in traffic is detected, and to be lengthened when the approach has a long queue. At this particular intersection, pedestrian phases are on recall which means the pedestrians get a walk phase every cycle. This means enough time for minimum walk and pedestrian clearance must be provided to every phase that includes pedestrians. The only vehicle phase that does not run concurrently with any pedestrian phase is east bound left (EBL). When EBL's detector is unoccupied the entire phase is skipped. If EBL is skipped and all other phases gap-out(end because a critical gap in traffic is detected) before pedestrian clearance is over, the minimum cycle length is 50 seconds.

NBT Vehicles in Queue
During the observation period the vehicle phases generally gaped out before the pedestrian phase was over. Therefore the pedestrian walk phase plus the ped clearance was the split used in the ring diagram for the north bound through (NBT) and west bound left (WBL) phases. The EBL phase was given a 10 second split because there was generally a car or two waiting for green. Had traffic volumes been higher a longer cycle length would be observed. During the observation period the longest cycle length observed was about 75 seconds. This is likely not the maximum allowable cycle length, a characteristic that could only be observed if volumes for all approaches were high. The 60 second cycle used for analysis was most common during the observation period.

Vehicle Delay

The volume data was collected by taking 15 minutes of video and counting the cars in each movement group. Using the volumes and the cycle data, an analysis of vehicle delay was conducted. To analyze delay the following assumptions were made: saturation flow rate: 1600 veh/hr/ln, lost time: 4 seconds.

The overall intersection delay was found to be 16.91 seconds giving it a HCM level of service of B. Intersections with a level of service that high are relatively uncommon. Most if not all drivers would find this delay to be very manageable. The delay was so short at this intersection drivers often did not notice the light had turned green at first.

Pedestrian Delay

Walk Signal @ Granite St.
Motorists are not the only street users that benefit from a snappy signal. Pedestrians also see significant delay reductions when compared to intersections that are not timed as well. There are three pedestrian crossings at the intersection, located on the east, west and north sides.  The east side crossing serves pedestrians walking north and south bound. This pedestrian phase runs concurrently with the north bound through phase (NBT). The north bound right traffic (NBR) has a red arrow at this time to avoid conflicting with the pedestrians. The north and west side crossings both run concurrently with the west bound left phase (NBL).  The minimum walk for all of the crossings is set to 10 seconds. 

Pedestrians and Bicycles During NBT
Pedestrian delay can be calculated much more easily than vehicle delay because pedestrians do not queue during red. The delay for all of the pedestrian movements is the same because the walk time (w) is the same for all, 10 seconds. Using a cycle length of 60 seconds the average pedestrian delay at this intersection is 17.6 seconds, an HCM level of service of B. Having a short pedestrian delay is not only important for saving time but also for safety. If pedestrians are delayed for a long time they are more likely to ignore the don't walk signal and try to cross the street. The pedestrian compliance rate at this particular intersection was very high.  

Bicycle Accommodations

Approaching Cyclists in Bike Lane
This intersection's snappy cycle obviously benefits bicyclists as well due to short delays. There are also bicycle treatments at the intersection that make bicycling easier, safer and more efficient.  There are wide bike lanes provided in all directions excluding Granite St.

For NBR bicyclist the bike lane is continuous meaning these cyclists experience no delay. The section of the bike lane that goes around the corner is protected by a rough buffer.  This buffer discourages right turning vehicles from encroaching on the bike lane.

Right Turning Bikes with Buffer

Bike Lane on Waverly St.


A short video clip showing a few cycles of the intersection is provided below. The camera is oriented south meaning the north bound through (NBT) traffic is traveling in the direction of the camera. The following observations can be made:

  1. At 0:45 a gap is detected after a long NBT queue is dispersed and green in quickly transferred to WBL.
  2. At 0:50  a gap is likely detected for WBL yet green is held because the concurrent ped phase is not yet over.
  3. At 1:10 No vehicle is detected at EBL and the phase is therefore skipped.

This intersection is a great example of how snappy signal controls are beneficial to all street users. By having the shortest possible cycle and giving phases green only when needed, delay is reduced for everyone. A level of service of B for both vehicles and pedestrians is exceptional. Having the pedestrian phase on recall as opposed to push button reduces pedestrian delay which in turn makes the intersection more pleasant and user friendly for pedestrians. The reduction in delay allows street users to arrive at their destination sooner and with less frustration. The engineers who designed the timing plan for this intersection clearly had efficiency for all street users in mind.

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