Boston Transportation Department
Signal Operations Design Policy May 2023
5 SIGNAL TIMING CONCEPTS
It is known that the goals of traffic safety and traffic capacity may conflict when
determining the number of phases for an intersection and decisions related to the
treatment of turn movements, pedestrian crossings, bicycle flows, and whether movements
will have permitted turn conflicts or have protected phases (i.e., no turn conflicts allowed).
To maximize efficiency of signalized intersections, BTD requires that traffic signal control
be designed for the minimum number of phases that are necessary to provide an
acceptable level of safety. Considering this, the traffic engineer must carefully select the
appropriate use of protected/exclusive phases. Intersections that experience significant
conflicts between turning vehicles and pedestrians and/or between turning vehicles and
through traffic, or that have restricted sight distance may require a protected/exclusive
phase, which is acknowledged to potentially degrade the overall intersection and network
MOEs.
Likewise, the goals of traffic safety and driver convenience may conflict when it comes to
arterial signal coordination. Coordination that facilitates traffic moving at a safe speed
through multiple intersections with little or no delay is desirable. Policies that aim to avoid
long intervals of unused green time may slightly degrade vehicular MOEs for arterial traffic,
but help improve safety for all users and often improve MOEs for pedestrians. While the
addition of a traffic signal is not a remediation measure for speeding, unused green time
provides an opportunity to give more time to pedestrians.
During the development of signal phasing and timing plans, in addition to peak hour
patterns, off-peak and overnight timing plans shall be developed. Additional plans may be
necessary.
5.1 Cycle Length an d Signal Coordination
Signal cycle lengths should be as short as possible to minimize unnecessary delays for all
modes while maintaining safety, minimizing queues, and providing necessary traffic signal
coordination. Half cycle lengths should be used whenever possible.
Where attainable, cycle lengths and signal phasing and timing plans should be designed for
a maximum V/C ratio of 0.95 for each approach during peak hours and 0.90 outside of peak
hours. An individual V/C ratio greater than 1.0 is not desirable, as significant queues will
develop. Depending on the surrounding context of the intersection, the geometry and size,
unavoidable vehicular volumes, and other constraints, exceptions will occur. In these cases,