Click to Select
Lab Exercises
Theory and Concepts
Professional Practice
Example Problems
Related Links
Exit Chapter
Signal Timing Design: Glossary

Use your browser’s "back" button to return to the previous page.

Actual Flow Rate-- The design flow rate, or the maximum flow that is expected to use the intersection. See the theory and concepts modules on peak hour volume, design flow rate, and PHF.

All-red interval-- Any portion of a signal cycle in which a red indication is observed by all approaches.

Approach-- The portion of an intersection leg that is used by traffic approaching the intersection.

Capacity-- The maximum number of vehicles that can reasonably be expected to pass over a given roadway or section of roadway, in one direction, during a given time period and under the prevailing roadway, traffic, and signalization conditions.

Change interval-- Identical to the intergreen interval.

Clearance interval-- Identical to the all-red interval.

Critical Flow Ratio-- The flow ratio of the critical lane group within a phase. The actual or design flow rate for the critical movement divided by the saturation flow rate for that movement.

Critical Movement or Lane-- The lane or movement for each phase, depending on how you choose to subdivide you intersection, that requires the most green time.

Critical Volume-- A volume, or combination of volumes, which produces the greatest utilization of capacity for the street or lane in question, given in terms of passenger car units per hour per lane or mixed vehicles per hour per lane.

Cycle-- A complete sequence of signal indications. Each phase has been serviced and the cycle is beginning again.

Cycle Length-- The time required for one full cycle of signal indications, given in seconds.

Delay-- The stopped time per vehicle (in seconds per vehicle), usually calculated separately for each lane group.

Design Flow Rate-- Identical to the actual flow rate.

Effective Green Time-- The green time that is actually used by traffic. Some lost time occurs initially while traffic responds to the green signal and begins to accelerate. Some time is also lost during the intergreen period as vehicles stop in anticipation of the next phase.

Flow Rate-- The rate, in vehicles per hour or passenger car units per hour, at which traffic is entering an intersection.

Flow Ratio-- The ratio of the actual flow rate to the saturation flow rate.

Green Interval-- The portion of a signal phase in which the green signal is illuminated.

Green Ratio-- The ratio of the effective green time to the cycle length.

Green Time-- The length of the green interval and its change interval, given in seconds.

Hourly Volume-- The number of mixed vehicles that traverse a given section of lane or roadway during an hour.

Intergreen-- The time interval between the end of a green indication for one phase and the beginning of green for the next phase.

Intersection Flow Ratio-- The sum of all the critical flow ratios--one from each phase.

Lane Group-- Any group of lanes. Lanes can be combined during the signal timing design process in order to simplify the calculations.

Legs (intersection)-- The portions of the intersecting streets or roadways that are within close proximity to the actual intersection.

Level of Service (LOS)-- A measure of the operating conditions of an intersection.  See the theory and concepts modules for more detail.

Lost Time--  The time during a given phase in which traffic could be discharging through the intersection, but is not. This is the period during the green interval and change intervals that is not used by discharging traffic.

Passenger Car Units-- A unit of measure whereby large trucks and turning movements are converted to passenger cars using multiplication factors. This allows you to deal with mixed traffic streams more accurately than if you had assumed all vehicles were created equal.

Peak-Hour--  The hour of the day that observes the largest utilization of capacity, or the hour of the day in which the largest number of vehicles use the intersection approach or lane of interest.

Peak-Hour Factor-- The ratio of the number of vehicles entering an approach during the peak hour to four times the number of vehicles entering during the peak 15 minute period. In the absence of field information, a value of 0.85 is normally used.

Pedestrian Crossing Time-- The time that is required for a pedestrian to cross the intersection.

Phase-- The portion of the cycle that is devoted to servicing a given traffic movement.

Phase Sequence-- The predetermined order in which the phases of a cycle occur.

Queue-- A closely spaced collection of vehicles.

Roadway Conditions-- The physical aspects of the roadway, such as lane-width, number of lanes, easements, bike lanes, shoulder width, and any other aspect of the roadway.

Saturation Flow Rate-- The maximum number of vehicles from a lane group that would pass through the intersection in one hour under the prevailing traffic and roadway conditions if the lane group was given a continuous green signal for that hour. This assumes that there is a continuous queue of vehicles with minimal headways.

Signalization Conditions-- All the various aspects of the signal system, including timing, phasing, actuation, and so on.

Split--A percentage of a cycle length allocated to each of the various phases in a signal cycle.

Traffic Conditions-- The qualities of traffic, such as traffic speed, density, vehicle types, and traffic flow rate.