The Traffic Signal Network
Signal Timing Basics
** DRAFT **
by Arthur J. Dock
One of the great mysteries to many people is how signals are timed. Lack of a basic understanding can lead to frustration. After all, it is easy to be annoyed when no simple answer is given to an "easy" query such as "fix the signal timing!" Ah, if it were only that simple...
This article is a very cursory introduction to signal timing. By no means is this meant as an engineering document, proposed standard, or any kind of gospel. This is intended purely to inform about some of the issues involved in traffic signal timing. As this is a complex subject, everyone is strongly urged to undertake serious study and consultation with trained professionals before undertaking any project involving actual traffic signal systems.
What It Takes
Firstly, we'll start off with a simple outline that steps through the process logically...
Non-attainment areas, providing coordination, and signal spacing
Yellow and all-red time
Natural Cycle Length
A Fly in The Ointment
A non-attainment area is a geographic region that has not met pollution goals and is therefore required to coordinate traffic signals. In the roughest sense, traffic signal coordination means (as much as practical) to time the signals in a manner that promotes traffic flow. In practice, typically, the attempt is made to provide progression which is where the signals turn green for approaching traffic driving at the progress speed (which, hopefully, is close to the posted speed limit). Unfortunately, this isn't as easy as it sounds.
Several factors go into determining progression:
Posted speed limit (which should be based on the 85th percentile speed or speed that 85% of traffic travels at or below)
One of the most likely places for good progression is in the Valley of the Sun (Phoenix, Arizona and surrounding communities). This area has a major-mile grid system with collector streets typically located every half mile. Based on this layout, the desired cycle length can be easily determined. This is because the signal needs to change relative to the time that it takes for a vehicle to travel from one intersection to the next.
For instance with one mile spacing between signals:
|Speed in MPH||Speed in FPS||Cycle Length*|
|(MPH * 5200 / 3600)||(5280 / FPS)|
* Actually, the cycle length can (in many cases) be a multiple (e.g. 1/2, 2 times, etc) and still work. Of course, oddly spaced intersections may result in bad arrival times that cannot accomodate traffic for both directions (i.e. one direction gets through but the other doesn't).
From the Table 1 it can be seen that the faster the speed, the quicker the signals need to cycle. What can be done when the speed limit changes along a stretch of roadway? Compromise... of which there is more to come because, as you will soon see, not every signal can be "fast enough" to fit into neatly defined cycle lengths.
How long does a pedestrian need to get across the street? Firstly, there is a walk signal to get started with. This typically lasts at least 4 seconds. This is followed by a flashing don't walk that needs to be long enough to finish crossing. So, depending on how wide the intersection is will vary how long the don't walk is. Also, there may be extenuating circumstances that require a longer time... for instance, elderly persons, disabled, or children are probably going to need more time. Typically 4 feet per second is used and 3 feet per second (or less) can be used for special cases. A typical intersection with two through lanes for each direction, a center left turn lane, and bike lanes on each side will have a roadway width of about 68 feet. This results in a crossing distance of around 58 feet (curb to middle of furthest travel lane distance). In this case, the pedestrian clearance time would be calculate to 19 seconds.
Yellow and All-Red Time
There is much debate regarding what is adequate yellow and all red time. However, for this article the values used will reflect a simple method based on approach speed (stopping distance) for the yellow plus clearance time for the red. At 45 miles per hour a 4.5 second yellow is probably reasonable. An all red time of 2.0 seconds for the previously stated intersection width of 68 feet is, again arguably, a reasonable value. Some agencies may vary on how they calculate this, but typical numbers should be very similar.
Natural Cycle Length
Believe it or not, we now have enough information to create a basic signal timing sheet!
* The green time is equal to the walk + pedestrian clearance time. This discussion could get real ugly technical, but for now just consider that the pedestrian signals and vehicle signals will operate together.
It appears as though all is well with our sample intersection! It can service both east/west and north/south in a nice fast 59 seconds. This natural cycle length (or minimum cycle length) looks very good. Taking a look back at Table 1 shows that this type of intersection could successfully operate in any of the listed cycle lengths with time to spare.
The spare time allows flexibility. For example, if the intersection in Table 2 were running in a 80 second cycle, there would be 21 seconds that could be moved as needed between east/west and north/south. This would allow the signal to be adjusted for situations such as heavy east/west traffic with light north/south traffic and so forth.
A Fly In The Ointment
All is well until... things change! If left turns are added, the intersection becomes wider (more pedestrian time required), or some other parameter change, the picture may not be so rosy. Lets add left turns to the above intersection and see what happens.
|Interval||EB LT||WB||NB LT||SB||WB LT||EB||SB LT||NB|
Due to the way signals typically time, the total is really only half of what it might at first seem. For example, eastbound left turn and westbound left turn can time simultaneously. Consequently, this timing shows a natural cycle length of 81 seconds. Well, we do have a slight problem... according to Table 1, this signal is too slow to fit into a 45 MPH progress speed. And even in 40 MPH progress speed cycle length there are only 8 seconds left to "play with" when making adjustments for varying traffic demands. If the left turn times were a little larger, this intersection would be in even more trouble.
Remember, this intersection has only two through lanes for each direction, a left turn lane, and bike lanes on each side. Imagine if this were a BIG intersection (e.g. three through lanes each direction, dual left turn lane, right turn lane, and bike lanes). Given that scenario, reality gets even uglier!
So, what cycle length is actually used? Like many things in life, it often comes down to the least common denominator. In this instance, the "worst" intersection can often dictate the cycle length for the other intersections. This, in turn, results in a less than ideal progress speed.
Another method is to divide groups of signals into zones. Within the zones, the traffic signals share a common cycle length. One downside to this is that when travelling from one zone to the next, stops are often the norm (i.e. the signals at the zone boundaries are NOT in step with each other due to their different cycle lenghts).
There are additional things that can make it even more difficult to keep the signals running at their best and cause grief for everyone.
Preemption (train, fire station, light rail, and transit buses - disrupt normal signal operation)
Transition (getting back in step - after preempt as well as after time-of-day changes)
Side-street delay (the longer the cycle length, the longer you wait for your green)
These issues will need to be dealt with in a future article as they could each be quite detailed discussions in and of themselves.
Some interesting results of what has been covered here:
Timing signals is not an easy proposition
Bigger isn't necessarily better for intersections
If numbers don't "add up" be prepared to compromise - that is the art
Hopefully this article has been helpful in describing some of the challenges regarding signal timing. Hopefully it will now be easier to see that, as the Rolling Stones once said "you can't always get what you want, but if you try sometime, you might just find, you get what you need..."
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