Traffic Calming should be Implemented on Low Volume Streets

In the summer of 2019, several residents of our street expressed interest in adding speed humps to improve safety.  Our street has over 20 children living on it, and studies have shown that speed humps reduce the likelihood by over 50% that children living on a street will be injured or killed by being hit by a car in the street.  

We contacted our city councilperson, Erika Strassburger, who reached out to the Department of Mobility and Infrastructure (DOMI).  We were told that DOMI only considers the implementation of speed humps on streets with traffic volumes of more than 1,000 cars a day.  We were told the reason was due to funding constraints.  

The PennDot Traffic Calming Handbook states:

As a minimum requirement, the ADT [Average Daily Traffic] should exceed 1,000 vehicles/day or the peak hour volume should exceed 100 vehicles for the roadway to be considered for traffic calming.

No justification is provided in the handbook for this ‘requirement’ and it is unclear if this handbook influenced DOMI policy.

I reached out to Karina Ricks, the director of DOMI, who stated that Pittsburgh does implement some traffic calming projects on streets with fewer than 1,000 cars per day.  However, since DOMI receives more requests for traffic calming than they are able to implement, the daily traffic criteria helps them to winnow down the list of projects for consideration.

Given the reality of limited funding, it is important that money spent on traffic safety go to where it provides the most benefit.  However, it is unclear whether average daily traffic volume is a good predictor of the safety benefits of traffic calming. A 2006 study of residential streets in Longmont Colorado showed no statistically significant relationship between average daily traffic volume and accident rate per mile.  This is the only study I was able to find examining residential streets. The other studies I found that show accidents increase with traffic were evaluating highways and arterial streets.

The following factors are likely more important for prioritizing need than traffic volume :

• Whether there have been crashes on the street, especially crashes that have resulted in injuries or fatalities.
• The number of pedestrians or potential for pedestrian traffic.  Sometimes there is no pedestrian activity because the road conditions are unsafe but pedestrians would use the street if safety improvements were made.
• The presence of children.  Streets near schools, playgrounds, libraries and museums should be prioritized.
• Nearby public transit infrastructure.  Being able to walk safely to bus stops and rail stations is crucial for a good public transportation system.
• Incidence of drivers travelling at unsafe speeds.  Sometimes the speed limit is higher than a safe speed.  Speed limits are 25 MPH by default in the city, but many of Pittsburgh’s streets were developed before the widespread adoption of the automobile. The design of these streets are often narrow and should have a lower speed limit.

Erika Strassberger informed me that several of the traffic calming improvements in her district planned for 2020 are at locations with a past history of crashes and injuries.  Prioritizing these is appropriate. Erika also suggested using the Our Streets App to identify areas for improvement.  The Squirrel Hill Urban Coalition and BikePGH use data from the app to advocate for street improvements.

I have written previously that more funding should be devoted to traffic calming.  Provided that sufficient funding is budgeted, there is a compelling case for implementing traffic calming on low volume streets.

Traffic calming infrastructure doesn’t just improve safety, it also changes how streets are used and the benefits they provide our communities. When streets are safe they are used more by non-motorists.  Pedestrians feel more comfortable crossing the street mid-block. Cyclists are more likely to ride, especially children. Children may play in the street. Streets are public spaces and should be available to and benefit everyone, including non-motorists.

Let’s first consider the trade-offs of implementing traffic calming on a street with high traffic volume.  Since there are a lot of cars, traffic calming imposes a time cost on a large number of people. The reduced speed may increase the number of cyclists willing to use the street, but otherwise it does not enable additional use.  

By contrast, on a low volume street, the time-cost to drivers of slowing traffic is less because fewer cars are slowed down.  The improved safety also enables a lot more potential uses for the street. Parents will feel safe letting a child bicycle on such a street.  Pedestrians may save time by crossing the street mid-block.

Therefore in some situations the case for traffic calming is actually stronger on low volume streets than on high volume ones.

PennDot should remove the guidance to only employ traffic calming on streets with a minimum traffic volume, as should DOMI.  Traffic calming funding should be prioritized where it will provide the most benefit. Ideally, funding should be increased so that safety improvements are evaluated for every street in Pittsburgh.  This could perhaps be done over a 15 year time period as streets are repaved.