TRAFFIC SIGNAL PROGRESSION

You have probably seen traffic signal progression and not known what it was. As you drive on a major street, the lights turn green as you come to them. The traffic on the major street forms into "platoons" of cars that pass through the intersections when the signals at those intersections are green. This sounds like a simple task to accomplish. With a one-way street and few arterial cross-streets, it is simple. But the following complications can make the calculations become quite difficult:

SIMPLE PROGRESSION ON AN ISOLATED ONE-WAY STREET

The isolated one-way street is the easiest kind of street to progress. There are no interactions that need to be considered, so setting up the signal timing relations is easy. The following factors determine the necessary settings:

Here is an example of a street with one-way progression, with a time-space diagram, and an animated view of the street. The horizontal direction of the time-space diagram runs along the street, and shows distance. Time flows upward on the time-space diagram.

one-way time-space diagram
animated one-way street

Figure 1: ONE-WAY STREET.
Notice how the platoons of cars arrive at each signal when it is green.

File size limitations on my website account for the jerkiness of the animation. Also note that in the sped-up animations on this page, the yellow periods would be too short to be visible.

PROGRESSION ON AN ISOLATED TWO-WAY STREET

When traffic is moving in both directions on the same street, it makes progression a lot more complicated. With two-way traffic, either the street must be progressed in only one direction, or a two-way progression that works must be found. Now, the signals must be placed at strategic points with respect to the distance traveled in one signal cycle. This is because the signals for both directions of travel must usually be green at the same time at the same intersection. The ideal spacing of signals is one half-cycle of travel time apart. This makes the signals alternate, so that when any one signal is green, the next signal is red. Thus, each signal is displaced one half of a cycle from the signals before and after it.

The alternating plan is the simplest of the two-way progression methods, and it has the highest bandwidth of any two-way plan. Here is an example of two-way alternating progression:

two-way time-space diagram
animated two-way street

Figure 2: TWO-WAY STREET - SINGLE-ALTERNATE
Notice how the platoons of cars arrive at each signal when it is green.

The following factors must be considered when designing progression for a two-way street:

Where the block lengths are so short that the shortened cycle lengths can't pass the traffic, the double-alternating plan is used. With this plan, pairs of signals show green at the same time, while the adjacent pairs show red. The signals are at the points labeled B. This allows two-way progression, but with a smaller bandwidth:

double-alternate time-space diagram
animated double-alternate

Figure 3: TWO-WAY STREET - DOUBLE-ALTERNATE

Note that, although the split between the two streets as shown is 50 percent (for ease in preparing the exhibit), the usual case for double alternate has a larger split for the progressed street.

The following colors are on the progression diagram:

LEFT-TURN SIGNALS AND TWO-WAY PROGRESSION

By reducing the through band, left-turn signals usually hinder progression. But if double-alternating progression is used, and provision is made to prevent yellow-trap, leading and lagging turns can make progression more efficient. Observe in Figure 3 above how the leading and lagging greens allow turns during the periods when only one through band is using the intersection. The signals are placed at the points labeled B.

At places where the through bands do not coincide, phasing designs can indicate split-phase operation, separated- phase operation (where other phases isolate in time the phase for one direction from the phase for the opposite direction), and dual-green operation (where the progressed street gets two green lights per cycle). While being inefficient, these phase patterns can solve otherwise unsolvable progression problems. They can be utilized as follows:

TWO-WAY PROGRESSION WITH UNEQUAL BLOCK LENGTHS

Unequal block lengths complicate the calculation of progression on a street. The best plan, if it can be accomplished, is to place the longest blocks in the places in the progression plan at those quarter-cycle points that are between the half-cycle points (the points labeled C). Otherwise, some signals might have to be removed and replaced with stop signs on the side streets. See the tips listed below.

PROGRESSION ON A PAIR OF ONE-WAY STREET

If the two one-way streets are a block apart, progression on the streets themselves is no problem, until the high-volume cross-streets are factored in. The low-volume cross streets do not need to be progressed between the two one-way streets, so the offset on the low-volume street at one one-way street needs no relationship to the offset at on the other one-way street. But major cross streets always pose a problem where they cross a pair of progressed one-way street, because:

PROGRESSION WITH A GRID OF STREETS

Depending on the grid of streets, progression over the grid can be easy or hard:

If one=way streets are evenly spaced and evenly loaded with even speeds, and the bandwidth can be less than 50%, then the four-blocks-per-cycle plan (Figure 4 below) can be used.

grid progression time diagram

Figure 4: GRID PROGRESSION - ONE-WAY STREETS

WHAT IF A PROGRESSED ROUTE MAKES A TURN AT AN INTERSECTION?

The street directions must alternate, with one direction being red while the other direction is green:

INTEGRATING A TRAFFIC INTERCHANGE INTO A PROGRESSION PLAN

Different kinds of interchanges require different progression methods:

TRICKS TO INCREASE PROGRESSION BANDWIDTH

Use these tricks to increase progression bandwidth:

  1. Try different timing splits between the progressed street and the cross street.
  2. Try different cycle lengths. Increase it if the system starts to back up at times.
  3. Try closing some streets, removing their signals, or flashing them during peak periods.
  4. A signal with left turn phases on both streets is a poor candidate for progressions. Try prohibiting and/or diverting some of the turns.
  5. Try lead-lag, split-phase, separated phase, or dual-green signal phasing patterns:
  6. Half signals can be used to simplify progression. A half signal is a signal that stops traffic in only one direction. Usually some movements must be prohibited to make a half-signal work. Occasionally two half-signals can be placed at the same intersection, with all crossing movements prohibited. A half signal can be used at any point in the diagram.
  7. Try splitting a high-volume two-way street into a pair of one-way streets. Often the number of lanes gained by this can result in allowing a lower cycle length or a smaller split.
  8. One trick often not thought of is to move the traffic on a major cross street onto a different street.
  9. If an intersection can't be integrated into the progression because of a very long or variable cycle length, let it float. Establish another progression system on the other side of the light.

WHAT NOT TO DO WITH PROGRESSION

Situations to avoid in progression situations:

  1. Don't try to force any speed other than the speed the traffic is actually moving at.
  2. Avoid left turn phases on both intersecting streets of the same intersection.
  3. Avoid intersections with more than 4 approach legs.
  4. Progression platooning is destroyed by any 4-WAY STOP, roundabout, or free-flowing interchange ramp merge.
  5. Avoid on-street parking and truck loading zones on progressed streets.
  6. Don't ignore high-volume special event discharges. Make timing plans for them.
  7. Don't let politics ruin a progression plan (e.g. favoring a special minor movement, such as city workers leaving work).
  8. Lane drops can destroy progression platoons during heavy traffic.
  9. Don't compromise a progression system to favor a transit route. It wastes the fuel of the other vehicles. Provide bus stops that don't disturb progression.
  10. If the progression system also runs at night, don't flash any signal red on the progressed street. Leave that signal operating.

TRY YOUR OWN CITY STREETS ON MY
PROGRESSION CALCULATOR.

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