Over the past month the Seattle Department of Transportation (SDOT) has been quietly testing their new Intelligent Transportation Systems program on the Mercer Street corridor. Usually shorthanded to “signal retiming”, the $13m Move Seattle project creates a dynamic corridor that adapts to traffic conditions in real time. 32 signals have been updated with the new software on Mercer, Roy, and Republican Streets from 3rd Ave W to Fairview Ave N.
The primary management imperative seems to be to shorten signal times in periods of heavy traffic, theoretically reducing the cascading gridlock that occurs as queues form during lengthy cycles and spill over into neighborhood streets. In times of heavily asymmetric traffic, such as the end of major Key Arena concert late at night, the system could keep eastbound lights green far longer, and make the lesser amount of north-south traffic wait. Anyone who has ever driven Mercer Street on a sleepy Sunday morning will attest that the current system has been rigid and static, slow to drive even when no one is on it if you miss the green-light wave.
SDOT’s early data shows significant improvement in travel time reliability for most drivers, significantly better eastbound flow, and slightly worsened westbound flow (see SDOT images below). Eastbound in the PM peak, when volumes are highest and delays worst, travel time has decreased by 2.7 minutes and average speed has increased by 1.4 mph. Doing a little math, for a 1-mile corridor this implies travel time improvement from 12 minutes to 9.3 minutes, and average speed improvement from 5mph to 6.4mph.
These are significant relative improvements, but put another way $13m just went to making peak car traffic go from barely faster than brisk walking to about half the pace of casual cycling. To the average observer, or to the driver whose sense of time is dilated by rage, the corridor will still feel very slow. SDOT estimates that the average peak driver will save about $4 per year in reduced fuel costs.
So signal timing can work to improve east-west vehicle flow. But when it does so, it must also impede north-south traffic, and all transit in the area is north-south. A total of about 13,500 vehicles use Mercer during the PM peak, and their prioritization can’t help but downgrade the priority of the thousands of peak commuters on Routes 1, 2, 3, 4, 5, 8, 40, 62, 63, 63, 70, 309, C, and the SLU Streetcar. If aggregate flow improves sufficiently that total systemic collapse is avoided, then maybe everyone actually will be better off. But even at its best, it tinkers at the margins at great expense. The fundamental space constraints that generate traffic will remain, and those who think tech will save us from traffic will continue to be disappointed.