Despite lower traffic volumes in 2020 because of the pandemic, many states across the U.S. have seen a year-to-date increase in traffic fatalities. National trends indicate speeding is a major contributor to this increase.
Higher driving speeds are related to more severe crashes because the kinetic energy involved in a crash is proportional to the vehicle’s speed squared. Higher speeds also are likely to lead to a higher frequency of traffic crashes — drivers travel a greater distance during the time it takes to perceive and react to a hazard the faster they drive. For pedestrians and cyclists, driving speed is the primary factor in crash survivability. Designing for low speeds (25 mph and lower) can substantially reduce the nearly 20% of fatal traffic crashes in which a pedestrian or cyclist dies.
Drivers generally use clues from the roadway design and the environment to choose their driving speed or take speed cues from other vehicles on the road. But when traffic is light, other factors can have more influence on how safe a driver feels traveling at a speed other than what’s posted. Such factors include roadway lane and shoulder width, the presence of a median, the frequency of driveways and intersections, the presence of a sidewalk or bike lane, and the distance between the edge of a road and hazards like steep slopes or trees.
When environmental cues and posted speed limits don’t match, it can lead to several safety concerns:
- General disregard for regulatory speed limits because of inconsistent speed limits for roadways with similar characteristics.
- Increased crashes due to insufficient time to react to hazards, such as curves or hidden intersections, when posted speeds are higher than the design speed.
- Increased criminalization of “normal” driving when posted speeds are artificially low for the roadway design and environment.
- Increased range of speeds driven on the same roadway, which can increase the likelihood of crashes.
A concept referred to as self-enforcing, or self-explaining, roadways focuses on managing driving speeds by designing roadways that provide clear clues on and around the roadway environment to encourage safe, appropriate speeds. One of the key features of a self-enforcing roadway is design consistency, meaning drivers can expect to see the same posted speed limits on roads with similar characteristics. Over time, drivers will intuitively recognize the appropriate operating speed based on the “feel” of the road. Of course, there are always situations that can’t be completely mitigated in the design process that require a lower operating speed to maximize safety. In these cases, advisory speed limits (yellow signs) can be used, but a safety review during the design phase can identify ways to minimize these situations rather than trying to mitigate them with signing and traffic control after the project is built.
An additional advantage of design consistency is that it aids autonomous driving programs. Computer drivers are worse than humans at interpreting mixed signals, so having the same driving rules and expectations in similar conditions will ease the transition to autonomous driving.
There are several tools available to designers to predict operating speeds and check for design consistency issues. Our team has used a suite of data analysis tools known as the Interactive Highway Safety Design Model on several projects for local transportation authorities, including the Buck O’Neil bridge replacement in downtown Kansas City, Missouri, and the recent Interstate 44 bridge replacements between Springfield and Joplin in Missouri.
Safer highway design is a critical element of reducing serious traffic crashes, but drivers must do their part as well. Drivers should remember to:
- Avoid distractions like texting, talking on the phone or eating when driving.
- Not exceed the posted speed limit.
- Find a safe place to rest or switch drivers when feeling drowsy.
- Monitor for bikes, pedestrians, motorcycles and other vulnerable road users and give them plenty of space.
- Slow down when visibility is poor or the roadway surface is anything other than dry and clear.
When we all do our part, we can create safer roadways for ourselves and our loved ones.
Whether you are rehabilitating existing infrastructure or developing new transportation networks, our innovative roadway solutions can help you improve critical connections to deliver safe, reliable, efficient travel experiences.