Cornell Local Roads Program

Accidental Thinking

Safety and the Travelling Public

When driving conditions get worse, there are more crashes. Due to snow and ice, winter driving is known to be more difficult than rainy conditions. Major storms this winter have shut down cities from Atlanta to Boston. Even in places that are used to winter storms, just one or two crashes during rush hour can cause severe problems. The New York State Thruway had large portions closed this winter due to adverse weather conditions.

While lower volume roads on the local system may not appear as critical, most of us live on a local road or street and just under half of all vehicle-miles are on the local system. A crash on one of these highways can keep us from getting to work, school, or our kid’s hockey game. A slippery ice sheet for playing hockey is good; a slippery road for driving is not.

3 people looking at a crashed UPS van
Figure 1. Accident on a rural highway.
Photo courtesy of David Hartman, Yates County Highway Superintendent

While we can do some highway design changes to improve highway safety, we also need to understand that there are limitations and that there are many factors that lead to highway crashes. The role of the driver is one of the most important. Let’s look at some of the critical issues and some new ways of thinking about highway safety

Crashes vs. Accidents

In the past, we would have called an incident an accident. Today we use the term crash. An accident implies fault or blame. While eventually there may be blame assigned for who is at fault, to improve safety we need to focus on the event and figure out what we can do to reduce the chances of it happening again. There are three factors that are important when looking at crashes:

Crash Types

Crashes are usually categorized by their severity and placed into one of three categories.

  • PDO – Property Damage Only – The least severe crash where no one is hurt. Modern vehicles are designed to take a lot of damage while keeping the passengers safe.
  • Injury – In this crash there is an injury to one or more people in the vehicle or a pedestrian or bicyclist outside the vehicles involved. The severity of the injury is not always reported and can vary quite a bit.
  • Fatal – In this case someone has lost their life. These are crashes we want to avoid.



crashed pickup
Figure 2. In 2012, 1,168 people were killed in traffic crashes in New York State


Probability is the measure of how likely it is that a crash will occur. Confusing or non-standard roadway alignment can lead to a higher probability. Putting up signs that do not follow the Manual on Uniform Traffic Control Devices (MUTCD) can lead to crashes, especially if the signs in question are non-standard. The U in MUTCD stands for uniform and good signage helps drivers know what to expect while driving down a highway.

Non-uniform sign
Figure 3. A non-standard curve sign does not provide the driver
with information on what would be a reasonable speed around a sharp corner.


The severity of the crash is critical in terms of what we do to improve safety. While counter intuitive, there are times when having more crashes may be considered acceptable. If a new signal at an intersection eliminated all of the injury and fatal crashes, having more property damage only crashes may be acceptable. While not always the optimum choice, with limited budgets we sometimes have to make compromises.

The table below shows how we might focus on more serious crashes even if they occur less frequently. Doing actions to reduce or eliminate fatal and personal injury crashes is usually the best bang for the buck. If the probability of a lower severity crash is high, it may be useful to focus on ways to reduce such crashes.

















By making spot improvements in the roadway you can move from the upper left toward the lower right in the table. Cells of the same background color may regarded as being comparable. For instance, by moving from medium probability personal injury to high probability property damage we reduce the chances that people will be injured.


Exposure is just a fancy term for traffic volume. The more traffic on a stretch of highway, the greater the exposure and the more likely a crash will occur. Many engineers will combine exposure and probability into a single factor to make tables like the one above easier, but this is not useful when dealing with very low volume highways (less than 400 vehicles per day).

car crashed on a rural road showinging a steep ambankment on the opposite sie of the road
Figure 4. Steep slope on a very low volume road

Humans vs. Vehicles vs. Highways

After a crash occurs, there are three general factors that are usually assigned some of the blame: the vehicle, the highway, and the humans involved (passengers and pedestrians as well as drivers). When fault is assigned, there can be interaction between these factors where two or all three are partially to blame. For example, the vehicle may be at fault if a tire goes flat. The highway may be considered at fault if a curve is banked incorrectly. If a tire blows just before entering a curve with an incorrect banking, both the vehicle and the highway may be considered factors in the crash.

By far the most common issue in crashes is the human factor. Distracted or impaired driving are the obvious factors, but inexperience, poor night vision, or driving too fast for conditions are common examples of human factors that may be involved in a crash. Combined with vehicle issues or less than optimal highway, human factors are at least partially or exclusively involved in 93 percent of all crashes.

graph showing factors contributing towards automobile accidents

Figure 5. Crash Factors

If we break down all crashes into which of the three general factors are involved, we can create a pie chart as shown in Figure 5 on page 8. Each wedge shows a portion of crashes with one or two factors involved. The small circle in the middle shows that 3 percent of all crashes involve all three factors (vehicle, highway, and humans) in some way.

Since human factors are involved in more than 9 of every 10 accidents, there is a lot of focus on trying to improve human behavior and provide more features in vehicles to alert drivers when there is a safety issue. For example, the National Highway Traffic Safety Administration (NHTSA) recently recommended back up cameras and alarms on all new cars.

The roadway condition, geometry, or other features on the highway are responsible for about one-third of all crashes (34 percent). Most of the time, there is some interaction with human factors, but many crashes could still be eliminated or made less severe by improving the highway. Properly signing sharp corners, removing hazardous fixed objects such as trees from the clear zone, and clearing vegetation to maintain sight distance are each examples of relatively low cost improvements that can help reduce the number (and severity) of crashes.

Modern vehicles are safer than ever and only involved in about 1 in 8 crashes (13 percent). Usually, there is some interaction with the driver or the highway. For example, if the vehicle gets a flat tire, but there is a good clear zone, the vehicle might safely pull over to the side of the road. If there is no clear zone, there may be a crash. A well maintained vehicle is less likely to be involved in a crash. Note that the wider clear zone benefits everyone and reduces the amount and severity of all crashes.

Human Factors

Since they are involved in so many crashes, let’s look a little closer at human factors. Human factors covers a huge range of different issues, but three in particular are critical to drivers and users of a highway:

Here are examples of why these issues are so important.


When driving on a highway, we have certain expectations; most vehicles are cars, particularly on low-volume roads. The signs are uniform across the country, and the geometry of the road (curves and widths) is consistent with our expectations.

large collection of intersection signs
Figure 6. Busy signage at an intersection

Figure 6 shows an intersection with lots of signs. Which is most important depends upon the needs of the driver. Did you see the hospital sign? We expect it to be white letters on a blue background, and a sign that does not meet our expectation may not be noticed until it is too late.


Ray LaHood, the former Secretary of Transportation, started a movement to address the issue of distracted driving by people using cell phones or texting while driving. In 2010, 6.6 percent of all fatal crashes involved distracted driving (phone, text, eating,  …). Driving is a complicated task and drivers need to pay attention to the task at hand. Even a few seconds of distraction can prove dangerous.

Reaction Perception Time

Studies show that drivers (on average) need about 2 seconds to respond even when paying attention. Texting adds up to 5 seconds of time before reaction occurs. What does that mean for distance travelled? Translating speed in mph into feet per second is shown in the formula below.





(5 seconds)


At 55 mph, those extra 5 seconds add over 400 feet to the distance travelled before you even react. That is the length of over 10 school buses!


While we use hearing and even feel to drive, driving is primarily a visual task and if conditions are not perfect, it makes the task much harder. As mentioned earlier, we expect vehicles to be cars. Vision comes into play when we have to look for people or vehicles that violate that expectation. Motorcycles and bicycles are sometimes involved in a crash because we expect a car and don’t see the smaller vehicle with only a single light. Glance at the picture below. How many workers did you see? There are actually 5 workers in the photo. Wearing a safety vest will make you more conspicuous and may save your life.

Accident site with 5 individuals some with hi-visability vests
Figure 7. Workers cleaning up after an accident

Doing Something vs. Nothing

There are lots of things road and street departments can do to improve safety and doing something to improve safety is a goal we can all benefit from. Small, inexpensive improvements in highways can go a long way to improving safety for everyone using our roads and streets. Look for those small improvements that enhance human factors and increase safety. Upgrade your signage, clear brush and trees particularly near intersections to provide consistent clear zones, and wear a safety vest when working in the right-of-way.

There is a lot more that could be discussed, but not in the space we have here. Below are some good resources to help get you started on improving highway safety and making your roads and streets safer for everyone who uses them.

Doing Something vs. Nothing

Doing something to improve safety is a goal we can all follow. Understanding that there are many factors in crashes helps us understand that small improvements can go a long way to improving safety on our highways. Rather than doing nothing because we cannot meet a ‘standard,’ we should look for those small improvements that take advantage of human factors and increase substantive safety.

There is a lot more that could be discussed, but not in the space we have here. Below are some good resources to help get you started on improving highway safety and making your roads and streets safer for everyone who uses them.


FHWA Highway Safety Program

AASHTO Manuals

Road Safety Fundamentals, Cornell Local Roads Program (pdf)

Low Cost Local Road Safety Solutions, ATSSA

NCHRP 321 - Roadway Safety Tools for Local Agencies

Fatality Analysis Reporting System (FARSs) Encyclopedia


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This work by the Cornell Local Roads Program (CLRP) is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 Unported License.