Thursday, July 17, 2008

How good can it get if traffic lights are optimized?

I summarize for you the results published in the most recent issue of the Journal of the Institute of Transportation Engineers (June 2008.)

After decades of neglect, the city of Balimore, Maryland decided to replace 1,300 old controllers and then optimize the traffic lights in the central business district and arterials that lead to and from it. The focus of detailed data collection and optimization was 425 intersections with traffic lights, of which 250 were in downtown and the rest on gateway arteries. Ten gateway arteries were identified numbering 175 traffic lights along their 30.5 miles of total length.

There are interesting similarities between Baltimore's past experience and Honolulu's current experience. I have been complaining that all traffic lights along Vineyard Boulevard run a very long 165 second cycle. That is exactly the old cycle in some of Baltimore's arterials.

What did this cost for the city of Baltimore? $762,500. How much did it save in delays, stops, fuel consumption, carbon monoxide and nitrogen oxides? $32,666,123. For a cost-benefit ratio of 43 to 1.

Delays (an engineering term for wasted time) dropped from 11,351 hours to 7,952 hours per day resulting in a value of time savings of $26,132,499. A drop of 30% means that a 20 minute trip on Honolulu arterials could be reduced to less than 15 minutes.

The article concludes as follows: "the benefits derived from this project proved that signal timings should not be compromised in the field, and an effort like this to perform city-wide signal timing optimization is well worth the money. The benefits outweighed the costs in less than three months and exceeded the expectations of city and public officials."

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