Friday, June 23, 2017

Introducing the 2017 NJ Complete Streets Design Guide

The Traffic Calming Ad Hoc Task Group of Princeton's Complete Streets Committee would like to draw your attention to the newly issued 2017 NJ Complete Streets Design Guide (PDF, 25MB). Princeton's municipal decision makers now have an authoritative guide to traffic calming measures that has the seal of approval of the NJ Department of Transportation and the Federal Highway Administration. 

What has changed from prior advisories from the state – and makes this document truly significant – is that the NJ DOT now prioritizes safety and place making rather than speed and throughput for car traffic

"Traffic calming is a system of road design and management approaches that balances vehicular traffic on streets with other uses. It reinforces the idea that streets should create and preserve a sense of place instead of solely facilitating vehicles passing through at the greatest allowable speed."

We propose that the municipality use this guide, moving forward, in the implementation of our Complete Streets policy. In fact, the guide codifies the ongoing efforts of town staff and committees to calm traffic in Princeton to improve the safety and accessibility of all streets, in all neighborhoods, for all users of the roadway.

Need for Traffic Calming in Princeton


Traffic calming is an important component of road safety, as many studies have shown that in a crash, higher motor vehicle speeds cause more serious injury and more fatalities, particularly when the crash also involves a person on bike or on foot. This chart from the design guide shows that doubling speeds from 20 to 40 mph increases the risk of pedestrian fatalities by a factor of 17.

This map highlights the high number of car crashes in general on our busiest streets as well as specific danger spots.

NJDOT Car Crash data 2001-2014 by Code For Princeton 

Bubbles indicate number of car crashes; original interactive map here.


Princeton is home to a variety of streets including municipal and county roads, state and federal highways. Speeding occurs on all of them, as evidenced by data from speed cameras. In the graphs, the red line indicates the posted speed limit; the blue columns indicate the distribution of recorded speeds.

Speedcam data provided by the Princeton Police Department

When considering the data, please remember that studies have shown that "Your Speed Is" readouts are effective at reducing drivers' speed. One quantitative study shows that cars' speed is 3-8 mph lower when a "Your Speed Is" readout is present, compared to when it is not. The speedcam data therefore shows Princeton drivers at their best behavior, that is, more closely abiding by speed limits.

2017 NJ Complete Streets Design Guide 

The NJ Complete Streets Design Guide will enable Princeton to implement its Complete Streets policy with the assurance that strategies covered by the guide are evidence based and endorsed by NJDOT and FHWA.

The design guide provides:

  • guidance on adopting and implementing a Complete Streets policy, public policy changes that can help facilitate implementation, and strategies for integrating Complete Streets into the planning and design process; 
  • a range of tools and treatment options that can be used to enhance a street's safety, mobility, access, and vitality. This includes guidance on ADA accessibility.

The guide also corrects two widely held perceptions that are traditional barriers to making streets safe and accessible for all users. As the guide notes, contrary to these perceptions:

Posting a speed limit does not necessarily slow down cars. Among the guide's prescriptions to slow down car traffic are narrowing car travel lanes ("road diet" and "lane diet") and using visual cues that make drivers want to slow down. The guide describes many more effective strategies to slow traffic. 

Reduced vehicular capacity does not necessarily lead to congestion. The guide introduces the idea of traffic evaporation, which means that when road capacity is reduced, vehicle volumes can actually respond by decreasing in similar proportion. In other words, "If you don't build it, they won't come". 

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