The effect of signal coordination on traffic arrival patterns is referred to as progression quality. Quantitatively, progression quality is expressed as the number of vehicles that arrive at an intersection approach while the signal indication is green for that approach, relative to all vehicles that arrive at that intersection approach during the entire signal cycle. This value is denoted as PVG, for Proportion of Vehicles arriving on Green. A general overview of signal coordination is provided in the rest of this section, as well as an example delay calculation for different arrivals rates during the green and red periods.
Fundamental Relationships The three most significant factors affecting progression quality are signal spacing, vehicle speed, and cycle length. The relationship between signal spacing and vehicle speed is most easily illustrated by considering a one-way arterial. Consider two intersections on a street running east-west separated by some distance do. With traffic traveling westbound, the time at which the signal phase of the westernmost signal (downstream signal) turns green after the easternmost (upstream) signal phase turns green should be equal to the travel time between the two intersections. The time difference between the start of the green between corresponding phases at adjacent signalized intersections is referred to as the offset, and is calculated as
= od offset
offset = start of green phase for downstream intersection relative to upstream intersection, for the same traffic movement, in seconds,
do = distance between upstream and downstream intersection for offset calculation, in feet, and
V = travel speed between upstream and downstream intersection, in ft/s. In practice, to take driver perception-reaction times into account, the downstream signal should turn green a few seconds before the lead vehicles reach the stop bar. For simplification, this time is not considered in the forthcoming example.