Measuring Traffic Delay From the Alexander Street Bridge Closure

Construction beginning to replace bridges on Alexander Street in Princeton, October 2019 (click to expand)

For several months, Princeton elected officials, business leaders, police and local media have been warning about what is assumed will be a minor disaster: the closure of Alexander Street to allow replacement of defunct bridges over the Stony Brook and D&R canal. The fear seems reasonable. Alexander Street is used for approximately 8,000 daily motor vehicle trips. If those vehicles cannot use Alexander Street, it is assumed that they will move to other roads, and cause traffic chaos there. But how bad is it really going to be? We set out to measure traffic before and after the closure of the road.

The Alexander St bridges are expected to be closed for about 6 months, starting November 6, 2019. If the road closure causes major regional traffic delays, we would expect that to mean much longer trip times for people trying to get into Princeton via Alexander Street. Using Google traffic data, which measures the speed of movement of millions of drivers using cell phone data, we recorded how long it took to drive from 5 locations near Princeton to Palmer Square on 14 dates in 2019. The measurements were taken in the morning rush hour (7.45 a.m. – 8.30 a.m.), on weekdays, when schools were in session, on days that were not public or religious holidays, were not during road work, and were not affected by serious bad weather.

Construction beginning on Alexander Road in Princeton in October 2019. (click to expand)

The trips that were selected involve the kind of journeys that regular commuters might make for getting into downtown Princeton. Three of them (starting from random addresses in Robbinsville, West Windsor, and East Windsor) are routes that typically use Alexander Street. In the event of problems caused by closure of the Alexander Street bridges, we would expect the time required to complete these journeys to get much longer. Two other trips that were measured (starting in Plainsboro and Hamilton) do not use Alexander Street as the quickest route. These journeys would only be expected to get longer if the closure of Alexander Street causes traffic to pile up on other roads leading into Princeton.

Example of a trip from West Windsor via Alexander Street to Palmer Square, Princeton, measured on Oct 25, 2019 (click to expand)

Trip times were recorded on seven mornings in May/June 2019, and seven mornings in October 2019. All of these dates are before Alexander Street is closed. The full data set can be downloaded here. We also saved screenshots similar to the one above for all of the trips that were measured. Although the trip times very from day to day depending on traffic / random crashes etc, for the most part they are fairly consistent. There was no significant difference in the average length of any of the five regional commutes measured between May/June and October. To try to measure the effect of the road work on locations near the work site, we also measured trip times in October from an address in the Canal Pointe Boulevard neighborhood to Palmer Square, which averaged 11.5 minutes.

Average trip times from five addresses near Princeton into Palmer Square before closure of Alexander Street in 2019. (click to expand)

After Alexander Street closes on November 6, we will continue to measure trip times, and look to see if they get significantly longer in the short-term (within 6 weeks of the road closure) and in the long-term (in the spring of 2020). Brave readers May want to use the comment box below to make a prediction about how many extra minutes of delay drivers are likely to face as a result of the Alexander Street road closure. Is it likely to be 2 minutes of delay? 5 minutes? 20 minutes? How much delay is required to officially qualify as “traffic chaos” or “a nightmare”?

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