Driving after dark: researchers at FHWA are striving to improve nighttime visibility, making roads safer for motorists and pedestrians

Public Roads, Jan-Feb, 2003 by Kenneth S. Opiela, Carl K. Andersen, Greg Schertz

It was 10:27 p.m., and John had been driving a winding rural road for hours. Although he had driven the road before and the traffic was light, the dark cloudy night and the pine trees flanking both sides of the pavement made it difficult to judge upcoming curves. As he approached one curve, he failed to notice a faded yellow-and-black sign until it was too late. Fortunately, his vehicle was going uphill, which moderated his speed, and the curve had a broad shoulder, so when his vehicle veered off the road, he was able to bring it to a stop just seconds before colliding with a tree. John was lucky.

The circumstances leading up to John's near-crash occur every night across the country. The challenges are greater than during the day because many of the visual cues necessary for safe driving, such as warning signs and pavement markings, are harder to see. As a matter of fact, approximately half of all fatalities due to motor vehicle crashes occur at night, even though the traffic volume is substantially less. According to data in Traffic Safely Facts 2000, published by the National Highway Traffic Safety Administration (NHTSA), the fatal crash rate for nighttime driving is three to four times that for daytime driving.

A large proportion of nighttime crashes are single-vehicle, run-off-the-road crashes, suggesting that a driver's inability to recognize delineation, guidance, or warning information may be a contributing factor. Lack of visibility may contribute to crashes involving pedestrians as well. In addition, adverse weather further complicates night driving by rendering pavement markings nearly invisible when covered with only a small amount of water, ice, or snow.

Research over the past two decades has led to a greater understanding of visibility requirements and spurred efforts to improve the night visibility of traffic control devices and the roadway environment in general. Despite the knowledge gained through research, encountering traffic signs or pavement markings that are barely visible at night is still not uncommon.

To provide State and local agencies with information on verifiable improvements that can save lives and reduce the cost of crashes to society, FHWA maintains an ongoing research program for improving the performance of retroreflective signs and markings, assessing the potential impact of new vehicle headlight technologies, analyzing the effectiveness of roadway lighting, and studying other aspects of human vision, visibility, and driver performance.

Reading the Numbers

Crash statistics can shed some light on the impact that time of day has on driver risk. An analysis of the Fatality Analysis Reporting System (FARS) data for 2000 reveals that: (1) 49 percent of all fatal crashes occur at night; (2) 81 percent of fatal crashes occur on dry pavement, both day and night; (3) 40 percent of all fatal crashes involve alcohol as a factor, with more than 60 percent of those occurring at night; and (4) problems with driver vision, vehicle hardware, or environmental conditions are cited as "related factors" in 15 percent of all fatal crashes. Since more than half of all fatal crashes involve only a single vehicle, the final statistic may greatly underestimate the impact of vision and visibility on driving safety.

The General Estimates System (GES), compiled by NHTSA, expands the numbers in the FARS to provide a national database of all highway crashes. For example, in 2001 an estimated 6.3 million highway crashes occurred in the United States, of which roughly 4.3 million involved property damage only, 2.1 million resulted in some injury, and 37,795 resulted in one or more fatalities (with a total of 42,116 deaths). The GES data reveal that more than 70 percent of all crashes occur during daylight. Thus, even though the number of fatalities during the day and at night are about the same, the percentage of fatalities at night is higher because there are less crashes and fewer vehicles on the roads. The percentage of crashes resulting in injuries also is slightly higher at night than the overall percentage of nighttime crashes. These statistics indicate that, on average, nighttime crashes are more severe than those that occur during the day.

Based on estimates of vehicle miles traveled, the overall nighttime crash rate is approximately 1.6 times that of the daytime rate, while the fatal crash rate is three to four times greater at night. Factoring out alcohol-related crashes, the nighttime fatal crash rate is still nearly twice that of daytime. Although other human factors issues, such as fatigue, may account for some of the increased risk, it appears reasonable that visibility (or lack thereof) also contributes to nighttime crashes.

The Good News

A single year's data provide only a snapshot of any given aspect of driving safety. An analysis of crash trends since 1975 shows an improvement in nighttime safety over time. Between 1975 and 2001, the number of fatal crashes during dark conditions declined steadily, especially compared with fatal crashes under other lighting conditions.