Wednesday, July 3, 2013

Signal Timing Progression Speeds in Downtown

A question on the Institute of Transportation Engineers listserv came from Boise, Idaho related to downtown signal timing progression speeds. Here's my response.
In Portland, OR our downtown progression speeds are 13 to 16 mph depending on time of day. We use a quarter cycle offset system. This works well because the block spacing is limited. I wrote a short blog post on this a few years ago that might be worth a read if you have the time.

In Baltimore, we did some work north of the downtown streets in 2007 where we chose a 25 mph speed in the off peak in an attempt to calm the traffic in the neighborhood where there were lower off peak volumes.

Chapter 2 of the Signal Timing Manual describes this rather succinctly. I have excerpted it for your benefit, but we tried to write a summary of how policies like this should be considered carefully with the context of the urban form. Here's some of the text edited for brevity:

Broader land use transportation policies should define the objectives for signal timing plans. User expectations for a street network are often the guiding principles for an agency’s policies. In a downtown setting, the City should want to make sure pedestrian safety needs are prioritized, which is best done by keeping travel speeds consistent. This is best done by having adequate pedestrian crossing times and the lowest cycle lengths possible given the width of streets.

I would not use a 85th percentile speed. I would strongly urge you to use a speed that is reasonable to deliver the safest system possible for pedestrians. Using 20 to 25 mph would result in speeds that result in a sustainable system. A slower speed such that we have used in Portland is also good for people on bicycles. Transit efficiencies are also gained if there are numerous blocks where buses are stopping every other intersection.

There was another answer from a retired engineer from Houston. This was very interesting so I am copying it here.

In Houston in the 1950's a fixed time signal system with variable frequency operation was installed. The CBD is a grid of streets (12 X 15 blocks) with 330 feet c.-c. of the streets. The off-peak signal operation was based upon 40, 60, and 80 second timing which resulted in 18, 20, and 22 mph speeds based on 1/4 cycle offsets. The traffic on streets both east/west and north/south traveled at these speeds. The signal cycles were determined by traffic counts entering and leaving the CBD on a major parkway from the west. In turn, a signal system frequency generator determined the cycle length from the traffic counts. This signal system operated this way up until the early 80's when it was basically utilized as fixed time system operated by time clocks for peak and off-peak operation with 80 second cycles.

One problem that existed in the afternoon peak was the problem of parking garages and lots emptying into the streets and causing a queue of stopped vehicles in all lanes before the next signal turned green, thereby causing the progressive speeds to breakdown. As a result, the signal operation was scheduled for simultaneous operation during the peak hour

We also had a freeway feeder street system of 4 streets by18 blocks (330' c.-c.) long that operated on fixed time of 40, 60 and 80 second cycles with the same 1/4 cycle offset and same speeds as in the CBD. The only problem that existed was the 2-way Main St. that caused all traffic on the 4 streets to stop. The traffic on streets both east/west and north/south traveled at these speeds. This system continues to operate today. Peak hour operation was simultaneous.

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