To reduce traffic accidents, we need to understand what causes them. The highway transportation
system can be broken down into three broad categories: the driver; the vehicle; and the road and
its environment. Factors that help cause accidents usually fit into one of those categories. Most
accidents have at least one contributing factor. Many have several.
Human factors include things like inattention or distraction, fatigue, alcohol use, and vision
problems. Vehicle factors may be mechanical failures, bad brakes or tires, or similar problems.
Road-related factors can be insufficient sight distance, poor or missing road signs, changes in
roadway width, or slippery road surfaces.
Figure 1 is based on studies of police reports of traffic accidents. The investigating police officer
lists factors that contributed to the accident. Driver errors are thought to contribute to most
accidents. A road condition is listed as a contributing factor 34 percent of the time, although it
may be more. A vehicle defect or malfunction is involved 12 percent of the time.
For highway department officials, the 34 percent of crashes where the road is involved is both
a problem and an opportunity. This is where lawsuits come from, but it also means that the
highway community has opportunities to prevent more accidents. In most road-related accidents,
the investigating officer also lists a driver factor. In other words, something about the road led the
driver to make a mistake, or the driver made a mistake, and the road did not allow for recovery
from the mistake.
Considering road transportation as a system, we see that there are things we can directly control
and things we cannot. Drivers and environmental events like weather are hard to control. If the
parts of the system that can be controlled (roads and vehicles) are designed to allow for those we
cannot (road users and weather), the system as a whole will work better.