Coordinating streets in both directions is much more difficult than coordinating them for one direction
The time-space diagram to the left shows a street with signals that are evenly spaced, with the exception that street 3 is at a half spacing. The dark green strip shows the through band for the east bound traffic, which is coordinated. The blue strip shows an attempt at coordinating the west bound traffic. The problem is that the west bound traffic will be stopped at street 3 and then again at 2, as shown by the light blue bands. You could change the offset on street 3 so that the west bound traffic does not have to stop, but then the east bound traffic would be stopped.
There are essentially two ways to cope with this dilemma. You can either find some compromise that coordinates both directions (the price is almost always decreased bandwidth), or you can simply ignore one of the directions and coordinate the direction with the higher flow. This is especially effective at rush hour times, where the inbound traffic can be coordinated in the mornings and the outbound in the evenings.
If it is necessary to coordinate both directions of traffic, there are a few options available. One would be to change the offset at 3 so that both east and west bound traffic can take advantage of the green. However, this greatly reduces the bandwidth in both directions.
Another option would be to both change the offset and increase the green given to the through traffic at 3. This option will expand the through bands in both directions, but the effects on the cross traffic may be too great to justify this move.
As you can see, there are several variables that can be manipulated to force the system to work in your favor. However, since you are working with a system each variable affects many aspects of operation. This is why it can be difficult to coordinate traffic in two directions.
The simplest way to ensure two way coordination is to use blocks and cycles of the same length and 50/50 phase splits. This will allow you to use an alternate system. Single alternate systems (see right) are so named because as you look down the street the signals alternate between red and green as shown in the figure to the left.
Double alternates work the same way except that you have two greens, then two reds, two greens, etc. From the diagram you can see that the bandwidth is half of the green time. There are also triple alternates, but the bandwidth is only one third of the green time.
Why would any one use these alternate systems that have such low bandwidth efficiencies? Well, depending on the block lengths, desired progression speed, and cycle lengths an alternate system may be necessary for all of your paramters to work together. For example, if you have block lengths of 500 feet, a desired progression speed of 35 mph (51 fps) and a desired cycle length of 45 seconds, then you can figure out your desired alternate system using the following equation with trial and error.
C = Cycle length (sec)
N = Number of alternates (1, 2, etc.)
D = Block length (ft)
S = Progression speed (fps)
For our example, a double alternate system gives us a 40 second cycle length at 35 mph. From the equation you can see that for faster speeds or shorter block lengths a higher number of alternates will generally be called for. For extreme cases you could have a simultaneous progression, where all of the signals are green at the same time.
When determining offsets for signals in a two way coordinated system, it is convenient to realize that, ideally, the sum of the offsets in the two directions should be a multiple of the cycle length. If this is not the case the green times will not align to form the best through bands. If you look at the picture of the single alternate system above you can see that the offset in each direction is half of the cycle length. The sum of those offsets is therefore one cycle length and all of the green time is utilized in both directions.
Although we have touched on progression throughout the coordinated signals materials, it may still be a somewhat vague concept especially since there are several types of progression. The animation below should help clarify the concept and illustrate the most common types of progression that can result from attempts at signal coordination.
Forward or simple progression is for ideal situations when there is little internal queuing and the green indications move down the system at the same speed as the traffic flow. As you can see from the animation, it looks like a wave moving through the system, away from the left side.
Simultaneous progression is used either when the block lengths are very short or the flow speeds are relatively high. The rationale is the same as with forward progression, that the signals will turn green as the platoon arrives from the upstream signal. However, since the travel time between signals is so short, it just makes sense to have them all turn green at the same time. Simultaneous progression can also be justified when there is internal queuing. If the internal queuing is at such a level that the clearance time is the same as the ideal initial offset the offset will go to zero. Simultaneous progression will either limit the bandwidth through the system or prevent the formation of a through band for the entire system.
Reverse progression is used when there is a high volume of internal queuing. The clearance times for each intersection are greater than the ideal initial offset, so the downstream signal must turn green before the upstream signals so that the queues can clear ahead of the through platoon. As in simultaneous progression, the bandwidth for the system will either be limited or there will be no through band for the entire system.
When different progression schemes are used throughout the day in response to differing traffic patterns you have flexible progression.
Now that you have reviewed the material on one way and two way coordination, you are ready to proceed to the Optimization page, where there are suggestions for determining the best way to coordinate your system.