Saturday, April 25, 2015

Signal Timing - the Phoenix Experience

One of the most difficult, challenging, hair-pulling, and rewarding experience is to completely re-time a city. Over the years, I have been blessed to be involved in many re-timing projects.  Timing a city provides a great benefit/cost ratio because for a small amount of money, you can gain huge user benefits.

When I say signal timing, I am not simply talking about settings in the controller - walk, don't walk, clearance, all-red, etc.  I am talking about creating cycle lengths and offsets for the safe and efficient movement of traffic along a corridor.  It's one of the most challenging projects a traffic engineer will ever undertake.

Normally, the process goes like this.  First, you collect traffic data along major corridors and at intersections.  Then, you will typically find logical progression corridors or grids.  The next step involves using signal timing software to optimize the timing.  Finally, these timings are loaded into the system.  Fine-tuning is necessary because as you change the system - you will likely modify the behavior of the motorists.  If motorists are suddenly faced with different timing, they might speed up or slow down - depending upon what they see.

I have seen some cities never perform major traffic signal timing projects.  For example, Phoenix is laid out in a grid pattern.  There are typically traffic signals every mile, half-mile and sometims quarter mile.  And it turns out that a 90 second cycle length provides incredible progression in all directions.  That is because it takes about 90 seconds to travel one mile at 40 miles per hour. If the capacity and phasing of the major intersections can be controlled with a 90 second cycle length - then this provides a perfect backbone for signal timing.  Simply time all of the intersections the same and put in a 90 second cycle length - you'll get pretty decent progression.

For years this worked in Phoenix - mostly because for a long time there were few left-turn phases.  But now that exclusive left turn phases are at almost every major intersection - the 90 second timing no longer works.  There isn't enough phase time to accommodate all the phases.  So even Phoenix now should perform signal timing on a frequent basis.

I am not sure how many cities perform signal timing overhauls.  As traffic patterns change and volumes increase, it can have a dramatic effect on the system.  In my opinion, these overhauls should be done every five years at least.  But budgets are tight, the work is difficult, and few people will recognize the difference.  It's not the flashiest project in the world. But the benefits are huge.