Forward and reverse progressions

Simple progression is the name given to the progression in which all the signals are set so that a vehicle released from the first intersection will arrive at the downstream intersections just as the signals at those intersections initiate green. As the simple progression results in a green wave that advances with the vehicles, it is often called a forward progression. It may happen that the simple progression is revised two or more times in a day, so as to conform to the direction of the major flow, or to the flow level. In this case, the scheme may be referred to as a flexible progression. Under certain circumstances, the internal queues are sufficiently large that the ideal offset is negative. The downstream signal must turn green before the upstream signal, to allow sufficient time for the queue to start moving before the arrival of the platoon. The visual image of such a pattern is of the green marching upstream, toward the drivers in the platoon. This is referred to as reverse progression.

Effective progression on two-way streets

In certain geometries it is possible to obtain very effective progressions in both directions on two-way streets. The existence of these patterns presents the facts that:

The system cycle length should be specified based primarily on the geometry and platoon speed whenever possible, to enhance progressions.
The task of good progression in both directions becomes easy if an appropriate combination of cycle length, block length and platoon speed exist.
Whenever possible the value of these appropriate combinations should be considered explicitly for they can greatly determine the quality of flow for decades.
In considering the installations of new signals on existing arterial, the same care should be taken to preserve the appropriate combinations and/or to introduce them.

Importance of signal phasing and cycle length

The traffic engineer may well be faced with a situation that looks intimidating, but for which the community seek to have smooth flow of traffic along an arterial or in a system. The orderly approach begins with first, appreciating the magnitude of the problem. The splits, offsets, and cycle length might be totally out of date for the existing traffic demand. Even if the plan is not out of date, the settings in the field might be totally out of date, the settings in the field might be totally different than those originally intended and/or set. Thus, a logical first step is simply to ride the system and inspect it. Second, it would be very useful to sketch out how much of the system can be thought of as an open tree of one way links. A distinction should be made among

streets that are one way
streets that can be treated as one-way, due to the actual or desired flow patterns
streets that must be treated as two-ways
larger grids in which streets interact because they form unavoidable closed trees and are each important in that they cannot be ignored for the sake of establishing a master grid which is an open tree
smaller grids in which the issue is not coordination but local land access and circulation

Downtown grids might well fall into the last category, at least in some cases. Third, attention should focus on the combination of cycle length, block length and platoon speed and their interaction. Fourth, if the geometry is not suitable, one can adapt and fix up the situation to a certain extent. Another issue to address, of course, is whether the objective of progressed movement of traffic should be maintained.