In mixed traffic, average transit speeds are slower than car traffic. Transit vehicles make regular stops, and tend to fall behind signal progressions.
On neighborhood streets and other small streets with high pedestrian activity levels, signals with short cycle lengths and progression speeds are favorable to lower-speed travel and let transit get back to the progression quickly. When transit has unpredictable dwell times, or where there is no coordinated signal progression, shortening signal cycle length can greatly reduce the time spent by transit vehicles waiting at red signals.
On streets without a coordinated signal progression, including streets that take a “minor” role in the signal system, shorter cycles reduce side-street delay.
On signalized streets with frequent transit service, in mixed-traffic or dedicated lanes.
Where active transit signal priority is less feasible or has limited benefits, including streets with short distances between signals, and downtown streets with high pedestrian activity.
Shorter signal cycles reduce net delay to transit vehicles, especially at near-side stop locations.
With shorter cycle lengths, the penalty to the transit vehicle for missing the green phase is smaller; additionally, shorter cycles generate shorter queues, reducing the likelihood that the transit vehicle will be blocked from reaching the stop by traffic.
Peter Furth & Joseph SanClemente. Near-Side, Far-Side, Uphill, Downhill: Impact of Bus Stop Location on Bus Delay. Transportation Research Record: Journal of the Transportation Research Board, No. 1971 (2006).
Short signal cycles reduce overall pedestrian wait times and cross street delay, improving rider access to transit. With sufficient pedestrian crossing time, shorter cycles can improve pedestrian safety by reducing wait times and crossings against the signal.
Shorter cycles may reduce the available time pedestrians have to cross the street. But pedestrian travel speeds vary widely, with mobility-impaired pedestrians traveling as slowly as 2 feet per second (f/s). Large pedestrian platoons, such as passengers alighting from a center platform, can also require more crossing time. Shorten crossing distances instead of lengthening signal cycles where possible. Pedestrians walking at 3 f/s can cross streets of up to 75 feet in 25 seconds, allowing evenly split 60-second cycles.
In an urban context, the minimum signal cycle length is determined by pedestrian clearance times and crossing distance (street width). Pedestrians moving relatively slowly, in the range of 2.5 to 3 f/s, should be provided enough time to cross when starting at the beginning of a WALK signal, even if clearance intervals are calculated for 3.5 f/s. A minimum WALK time of 7 seconds is recommended, with a 4 second minimum required. (MUTCD 4E.06).
Except where crossing distances exceed 70–80 feet, or where it is necessary to accommodate a third signal phase to separate turning vehicles from pedestrians, it is usually possible to set signal cycle lengths at 60 seconds.
On streets with neither a signal progression nor active transit signal priority, each signal cycle should include enough time to allow a bus to move from one stop to the next.