This page is meant to guide you through the design process. Knowing
a little more about how these various concepts are used together will make the individual
concepts easier to understand.
Pre-Design Data Collection:
The design of signal timing schemes is a fairly simple, though multi-step
process. First, you need to know most of the roadway conditions surrounding the
intersection you are working on. This includes the number of lanes, the width of the
lanes, the width of the intersection, the width of the shoulders, and more. Second,
you need to have information regarding the composition of the traffic, such as the
percentage of busses and the percentage of trucks within the traffic stream. You also
need to know the peak hour volumes and peak fifteen-minute volumes for all of the various
movements.
The Design Process:
The basic steps in the design process (assuming you are using Webster's method
see Cycle Length Determination module) are listed below. While this particular
listing is oriented toward Webster's method, most of the other methods incorporate the
same concepts, but in a slightly different way.
- Decide on a phasing plan.
- Calculate the length of the intergreen period for each phase of your cycle.
- Calculate the minimum green time for each phase based on the pedestrian crossing time.
- Calculate or measure the saturation flow rate for each approach or lane.
- Calculate the design flow rate for each approach or lane using the peak hour volume and
peak hour factor.
- Find the critical movements or lanes, and calculate the critical flow ratios.
- Calculate the optimum cycle length.
- Allocate the available green time using the critical flow ratios from step six.
- Calculate the capacity of the intersection approaches or lanes.
- Check the capacities/design flow rates and green intervals/minimum green intervals.
Adjust your cycle timing scheme if necessary.
Even though this outline is tailored for Webster's method, you'll find that most of the
other design methods involve many of the same calculations. Refer to this roadmap
frequently as you proceed through this chapter, so that you can see how each calculation
is related to the design process.