Six months ago, newspapers told of impending doom due to the closure of the Alexander Road bridge for a six-month repair project. For some, the result of the project has been the “traffic hell” that we warned of in our headline. One resident wrote a letter to the editor saying that he and his wife were almost late for a surgical procedure after it took nearly an hour to drive from Princeton Junction to Bunn Drive.

Now, as the project is expected to be completed later this month, the question is: Was this experience widespread?

Sam Bunting, a walking and biking activist, decided to answer this question with a mini scientific study of sorts, to see what the implications might be for road policy more broadly. As an associate professor of molecular biology and biochemistry at Rutgers, Bunting is no stranger to crunching numbers.

In a post on Walkable Princeton, a blog that Bunting runs together with another Princeton resident, Bunting described the study and its outcome:

Traffic counts show that Alexander Street is used for over 7,000 motor vehicle trips per day, so many experts predicted that closing the road would cause major problems. The chief of police in Princeton said that roads would be “very congested … we want people to understand there are going to be delays during rush hour.” But just how bad did the delays get? We measured travel times before and after the construction project began, and found (perhaps surprisingly) that most Princeton commuters are experiencing delays of three minutes or less on trips into town during the morning rush hour.

In 2019, before Alexander Street was closed, we measured how long it took drivers to get from a number of addresses in local towns to Palmer Square, in the heart of downtown Princeton. Travel times were measured using Google Maps’ traffic layer. Most drivers have cell phones in their vehicles, which are constantly sending signals to nearby cell phone masts. That means that companies like Google can track the location of cell phones and precisely measure the flow of traffic. We made screenshots of travel times at 8 a.m. on at least six days when school was in session, which were not holidays or impacted by sudden bad weather or emergency construction work. A total of 166 trip time measurements were recorded. Trip times were the same in May and October of 2019, before Alexander Street closed. But what happened when the construction began?

We considered three randomly selected addresses for which the fastest route into Princeton would normally involve driving on Alexander. When Alexander closed, we expected these trip times to get longer because commuters would no longer be able to use the quickest way into Princeton. Trip times did get longer in each case — but only by about two or three minutes.

If drivers that normally used Alexander Street to get into Princeton made a detour because of the bridge replacement project, did that cause problems on other roads leading into town? We tried to test that by considering trips from other addresses for which the fastest route into downtown Princeton does not usually involve Alexander. For these trips, drivers would normally use Princeton Pike, Washington Road, or Route 27, and might expect delays if traffic from Alexander Street was displaced because of the construction project. But there was no evidence at all that displacement of Alexander Street traffic caused delays for these trips. Travel time was the same before and during the bridge replacement project:

For a small number of drivers who make local trips along the Alexander Street corridor, the bridge replacement project caused a bigger impact on travel time. We considered the time taken to commute from an address in the Canal Pointe Boulevard development into downtown Princeton. For this trip, the only sensible route involves driving up Alexander Street. Delays were more like six minutes per trip, presumably reflecting the fact that these drivers were forced to make a bigger detour to find an alternative route into town:

It is slightly surprising that delays caused by the Alexander Street bridge project are so small. Even after closing one of the busiest roads in Princeton, many commuters are not seeing big differences in travel times, and those who are impacted are usually experiencing delays of three minutes or less. People are still definitely experiencing bad traffic at rush hour, but most of that traffic was there before Alexander Street closed. One possibility is that drivers were mostly able to adapt to the closure of Alexander Street by adjusting their route, the time they drove to work, or even by switching to mass transit. Mayor Liz Lempert and other officials from the town worked extensively with the state Department of Transportation and local business leaders to try to help drivers cope with the closure of Alexander Street.

Although our study rules out the idea that the Alexander Street road closure caused widespread, crippling delays, it is possible that there were significant traffic impacts for a subset of trips into or out of town. For example, anecdotally, delays in the evening rush hour have been worse than in the morning rush hour. A broader survey based on real-time traffic data might reveal such a difference. Shortly before it was disbanded by Council, the Princeton “Complete Streets Committee” was exploring such approaches with traffic expert Thomas Brennan. We also don’t yet know if there has been an increase or decrease in crashes since Alexander Street closed.

But for the most part, the closure of Alexander Street has been a challenge that Princeton seems to have taken in stride. As the chief of police now says, “the overall traffic volume impact on our local roads has not been as bad as we expected”.