Roadway Design is Often Responsible for Driver Error

Woman riding bike on street with car parking and 20 mph speed limit and "Neighborhood Greenway" signs.

By Beth Osborne 

A research paper published in the Journal of Planning Education and Research finds that, while conventional wisdom attributes most roadway crashes to driver error, it is the characteristics of the roadway and built environment that are the primary  causes of driver error.  This finding breaks with widely-held beliefs in transportation (based on a study over 40 years old) and offers engineers and DOTs a more active role in reducing crashes through engineering and design.

The authors of “Toward Safe Systems: Traffic Safety, Cognition, and the Built Environment” found that drivers do not make choices through conscious,  intentional decisions but instead  through the inference of expected behavior based on the roadway type. For example, if the road is narrow and flanked by Main Street-style development, people are more likely to drive slowly and expect pedestrians. But if the roadway is wide and flanked by development set back from the road by a large parking lot then people will drive as though it is a highway.

The report explains that the design of the built environment primes drivers’ expectations about potential conflicts or hazards and to what risks they should be prepared to respond. The embedded conditions in the built environment can cause “inattentional blindness”: situations in which drivers look but still do not see a conflict type that the design doesn’t prepare the driver to watch out for (usually pedestrians, bicyclists or motorcyclists).

Design cues can also lead drivers to break laws designed to protect safety, such as the speed limit.  The report cites another study that shows drivers will adopt the operating speed (even when it is in excess of the posted speed limit) because the driver goes the speed that feels safe in the design environment before them. If that leads to a crash, it is attributed to reckless behavior as opposed to the design of the built environment that induces the behavior.

Overly complicated and difficult-to-navigate roadway design can be unintuitive (especially for those unfamiliar with the area) and thus lead to design-induced driver error. Complex intersections or interchanges between freeways and arterials can be difficult for drivers to interpret and determine safe or appropriate behavior. However, again, the errors caused by that complexity is often blamed on the driver.

The report authors suggest that transportation and development agencies need to better understand the connection between design and behavior and how this connection leads to more crashes and fatalities. They propose psychological studies and simulations to identify the environmental features that most influence behavior.

The authors also point out that “if latent errors are indeed a primary determinant of the incidence of traffic-related death and injury, then planning and policy decisions regarding the location and configuration of future development may have profound long-term impacts on traffic safety.” That would mean that development plans, zoning codes, building permits, and roadway design all have safety impacts. The report specifically cites the safety effects of performance measures, like level of service, that encourage the addition of lanes and faster speeds in the wrong context. Such approaches may be considered conventional and even the default approach to roadway design, but they can have negative  safety consequences that, while the study does not address it, could have significant implications for liability surrounding crashes and fatalities.

Finally the report proposes a crash forecast model based on the conditions they studied in Charlotte-Mecklenburg County, North Carolina (the focus area for the study). The authors  note that the local conditions are reasonably common nation-wide allowing the model to be used broadly. Such a tool would be extremely useful in predicting and preventing the conditions that lead to crashes and could be incorporated into new roadway and roadway maintenance design and planning.

Photo credit: Places for Bikes via Flickr, unmodified. License