Speedy Road Design Undermines Safety

This guest post comes to us from Chris McCahill, a senior associate with the State Smart Transportation Initiative.

Despite gradual improvements, the U.S. is falling behind its peers in terms of traffic safety. Making matters worse, our nation’s most vulnerable road users—pedestrians and cyclists—make up a growing share of traffic fatalities in recent years. In response, the U.S. DOT has made bicycle and pedestrian safety a high priority, state laws are beginning to address the needs of non-motorized road users, and many cities are installing new bike facilities and stepping up traffic enforcement.

All of this, however, is being done within a framework that has for decades prioritized high-speed travel—arguably one of the greatest obstacles to pedestrian and cyclist safety.

This prioritization of speed has played out in many ways, but particularly in the design process. The conventional approach encourages engineers to design roads for higher speeds than drivers are actually meant to travel (through the use of a design speed). This often leads to more gradual curves and wider roadways with fewer objects near the edges, which make it easier for drivers to travel fast. More recent design guidelines, however, recommend setting a low design speed, sometimes called a target speed, and using design cues to promote low-speed travel wherever it makes sense to do so. Many cities and a few states have begun to recognize and adopt these newer guidelines, but there’s still a long way to go before low-speed design is a common practice, rather the exception.

The highway mentality

The conventional approach to road design grew out of an era when the nation was building thousands of miles of rural highways connecting its major cities, on which drivers could travel faster than ever before. To accommodate this, the American Association of State Highway and Transportation Officials (AASHTO) recommended, until quite recently: “every effort should be made to use as high a design speed as practical.” This approach, sometimes called “forgiving design,” ensures that drivers can see far ahead and makes it less likely that they’ll run off the road or into oncoming traffic, even if they briefly lose control.

While this approach has helped reduce the number of crashes on highways in rural areas, it has also become a common practice in urban areas and in places with high levels of non-motorized road users. This ultimately encourages high-speed travel in locations where speed poses serious risks: narrowing drivers’ fields of vision, making it difficult for them to react quickly, and drastically increasing the risk of injury to someone who is struck. As the League reported earlier this year, a majority of cyclist deaths now occur on urban arterials, which are systematically designed for high speeds but often run through areas with lots of activity and various kinds of roads users.

Emerging best practices

Fortunately, nearly every professional design organization now recognizes that higher design speeds are not always appropriate for every location. In 2004, AASHTO published A Guide for Achieving Flexibility in Highway Design—a companion to their preeminent design guide (the “Green Book”)—which states:

“Given the historic equating of design speed with design quality, the notion of designing a high-quality, low-speed road is counter-intuitive to some highway engineers. Yet it is in many cases the appropriate solution to a sensitive neighborhood or other street design problem. The severity of pedestrian crashes, a significant concern in urban areas, is greatly increased as speeds increase.”

The most recent Green Book, published in 2011, also alludes to a more nuanced approach in how design speed should be used, stating that on “lower speed facilities, use of above-minimum design criteria may encourage travel at speeds higher than the design speed.” 

In the past decade, other professional organizations have produced their own guidelines that address the design speed issue explicitly. For example, in its 2010 publication, Design Walkable Urban Thoroughfares: A Context Sensitive Approach, the Institute of Transportation Engineers (ITE), recommends using a target speed, which it defines as “the highest speed at which vehicles should operate on a thoroughfare in a specific context.” It also calls speed “the most influential design control, and the design control that provides significant flexibility in urban areas.” The National Association of City Transportation Officials (NACTO) also encourages the use of target speed in its Urban Street Design Guide.

Reforming design standards

Many cities and a growing number of states have begun to endorse these newer guidelines. This is important because it signals to designers that they should have those guides on hand and know how to use them, but it is only a first step. Unfortunately, the official standards in many states still discourage or preclude low-speed design. In 2010, the ITE Journal reported that 12 states recommend using design speeds that are five to ten miles per hour above the posted speed limit, while 24 states require a minimum of 11-foot wide lanes and six states require 12-foot lanes. These standards generally apply on all state roads and are often adopted by local agencies.

 Reforming design standards can be a major undertaking, particularly at the state level, but a handful of agencies have proven that it can be done. In 2006, the Massachusetts DOT published a completely revised design guide that explains: “the design speed should be a logical one with respect to the target speed and existing operating speed.” In 2008, the DOTs from New Jersey and Pennsylvania jointly published the Smart Transportation Guidebook, which recommends choosing a “desired operating speed,” similar to a target speed, that should “for most roadway types be the same as the design speed, and also the same as the posted speed.”

In addition to treating design speed differently, these guidelines give engineers greater flexibility and encourage them to think about roadways as more than conduits for high-speed vehicle movement. In each case, the new guidelines eliminated key obstacles to creating safer, lower-speed roads. Moreover, agencies interested in reforming their own guidelines can now turn to those involved in earlier efforts for guidance and inspiration.