Human Factors in Transportation Cases—Roadway Design

Human factors experts often analyze the impact of the design and condition of a roadway environment on a driver’s performance. Investigations include determining whether the traffic control devices, pavement markings, or signage were visible, whether they were consistent with a driver’s expectations, whether they caused the driver to take a particular action, and whether they were sufficient to prevent a collision.  For example, a human factors expert may investigate whether there was sufficient information available to the driver on the roadway to avoid the collision, whether that information was available in sufficient time to avoid the collision, and/or whether additional signage or traffic control devices would have prevented the collision.  In other words, human factors experts use the concepts of positive guidance and expectancy to evaluate the adequacy of traffic control devices and the roadway design.

When driving, road users seek out information in the roadway and surrounding environment in order to successfully navigate to their destination, select their vehicle’s path, and control their vehicle (e.g., steering, braking, etc.). Ninety percent of drivers’ tasks consist of seeking out and obtaining visual information from the roadway to maneuver their vehicle safely. In order for drivers to make safe decisions while driving, the visual information available to the driver must be adequate, accurate, and not be confusing.[1] To ensure that drivers have sufficient visual information to safely navigate roadways, two human factors concepts are employed when designing roadways in the United States—positive guidance and driver expectancy.

Positive guidance is the concept that drivers can safely avoid hazards when the roadway environment provides sufficient information where/when it is needed and in a form that is easy to understand.[2] Positive guidance is provided to drivers through traffic signs, pavement markings, traffic control devices, and perhaps most importantly, by the view of the road ahead.  Expectancy relates to a driver’s readiness and ability to respond to situations, vehicles, and information that they encounter on the roadway in predictable and successful ways.[3]  This includes a driver expecting traffic control devices and other drivers to operate in accordance with established conventions so they can respond safely and appropriately. When drivers’ expectancies are violated or when positive guidance is not provided, drivers may respond more slowly, incorrectly, or not at all.

As shown in Figure 1 below, the traffic control devices in this work zone provide positive guidance to drivers so they can safely navigate the work zone. The advanced warning signs and traffic signs provide information on the road conditions ahead; the traffic light tells drivers when they must stop and when it is safe to proceed; the “stop line” sign informs drivers where to stop their vehicle when the light is red; and the pavement markings barrels indicate which lane is designated for travel and guide their path. 

As a result of this setup, drivers who encounter this work zone configuration can reasonably expect that it is safe for them to enter the work zone when the temporary traffic light is green and it is safe for them to proceed through the work zone in their designated travel lane.  They can also reasonably expect that there will be no oncoming vehicles approaching from the opposite direction in their designated lane of travel while they are inside the work zone area. 

Work Zone Setup in Mountainous Area

Figure 1. Example work zone setup

If the work zone design includes uncontrolled entry points (e.g., driveways, intersecting roads) that do not have traffic control devices and/or signs to inform a driver when it is safe to enter the work zone or which direction traffic is traveling, drivers who encounter the work zone at an uncontrolled entry point may not have sufficient information to determine if it is safe to enter the work zone or if it is safe to travel in a particular direction in the work zone.  In work zones that fail to provide adequate positive guidance and/or violate drivers’ reasonable expectancies, conflicts and collisions between vehicles are likely to occur. 

If your case involves roadway design issues, contact Dr. Nancy Grugle to discuss how human factors issues may have played a role in the collision.


[1] Human Factors Guidelines for Road Systems. (2012). NCHRP Report 600, 2nd Ed. Transportation Research Board, Washington, DC.
[2] Eugene Russell, “Using Concepts of Driver Expectancy, Positive Guidance and Consistency for Improved Operation and Safety,” Transportation Conference Proceedings, 1998.
[3] “Driver Expectancy in Highway Design and Traffic Operations,” Report No. FHWA-TO-S6-1, May 1986.