Are Speed Limit Signs a Cost-Effective Strategy to Manage Urban Speeds and Implement 20MPH "Living Streets"?

According to the 20's Plenty campaign, 20mph speed limits for all residential areas throughout Europe have emerged as the key recommendation of the EU Transport and Tourism Committee on improving road safety in Europe. This announcement was made by European Parliament on June 21, 2011.

Studies have proven that 20 MPH speed zones have a dramatic impact on reducing the level of pedestrian injury. Slower street speeds are also associated with benefits such as more social connections, a stronger sense of community, reduced noise and stress levels, higher property values, more accessible walking and better conditions for bicycling. Unfortunately, citywide traffic calming projects, such as those created in many European cities over the past 30 years, can take many years or even decades to roll out across an entire city. So how can cities move forward on creating these zones in a more cost-effective way?

A British review suggests that, when combined with community involvement, even just posting lower speed limits can be an effective method to shave a few miles per hour off the speed of local vehicular traffic:

"Where signs-only schemes are used, small speed reductions and accident savings can be achieved if associated publicity and enforcement campaigns are also used."
Source: UK Transport Research Laboratory Report 363 Urban speed management methods

The British study looked at impacts of signs-only schemes on traffic speeds, concluding that there are small reductions of speeds just from signs particularly when they are combined with community awareness, enforcement and/or publicity. Although the decreased speeds resulting from signage efforts pale in comparison to the decreased speeds that communities see on so-called "living streets" with traffic calming (also known as "self-enforcing" speed limits), it is interesting that they did find reductions from signage alone.

Some cities and towns already seem to be aware of this strategy. The 20's Plenty campaign points out that cities in England with a population of 5 million people have adopted 20 MPH zones as their official policy. Locally, many cities and towns in Connecticut post 15 MPH or 20 MPH speed limit signs within residential areas, and even New York City is now cutting speed limits to 20 MPH within 75 different neighborhoods across the city. In Hartford, Connecticut, a 10 MPH speed limit is posted in front of the Legislative Office Building, and helps protect the lives of our state legislators.

Yet, in New Haven, our traffic engineers continue to post speed limit signage of 25 or 30 MPH across the entire city. The high speeds that result contribute to hundreds of severe injuries per year on our city streets.  Slower posted speed limits in residential areas, hospital, university, and school zones are something that the New Haven Safe Streets Coalition has specifically called for on many occasions.

Cities must be realistic when creating a strategy for more livable streets. On one hand, small decreases in speeds may not meet overall community goals for speed management, and probably won't significantly reduce the number of crashes.

But on the other hand, looking at the graph at the European Cyclists Federation website and the Federal Highway Administration data here, it seems that even a small reduction in speed within a given area could have some impact on the severity of injuries and fatalities over time.

Speed limits and local governance

One of the key limitations of the Transport Research Laboratory report, in our opinion, is that it did not address the question of whether lower speed limits in and of themselves can, over time, create additional political will for additional "self-enforcing" (traffic calmed) streets.

Signage for reduced speed limits are an "advertisement" of sorts for more livable streets within a city. Furthermore, it may be easier for neighborhood advocates to argue for concrete changes if they are able to point to even bigger disparities between desired outcomes (livable streets) and the day-to-day realities of vehicles traveling at dangerous speeds through their neighborhood.  Slower speed limits also provide ammunition for local residents to use more informally, when they talk to their neighbors who clearly violate the laws.  The converse principle is also true: treating all roads equally - by posting all of them at 25 or 30MPH - sends the implicit message that little can be accomplished, and discourages residents from taking any action.

Either way, the evidence seems clear that posted signs can translate into slightly lower speeds in the short term.  By catalyzing more effective citizen advocacy, can they also lead to the implementation of more effective, and more widespread, traffic calming strategies in the long term?

Update, 8/31/12: Portland, Oregon will be reducing speed limits to 20 miles per hour on 70 miles of residential streets.  Even a small adjustment in speed limits on this many streets is likely to save lives.  The results of this conversion will hopefully become a "case study" for complete streets advocates in other U.S. cities.