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Understanding Highway Crash Data

In order to discuss the issue of highway fatalities, it is crucial to understand the differences between the terms "fatalities," "fatality rate," and the "fatal accident rate."

  • Fatality figures -- a simple tally of the number of people killed in automobile accidents -- are the least useful criteria for analyzing highway safety trends. Reports and studies based on the numbers of fatalities have little merit or meaning within the context of highway safety trends.
  • The fatality rate, which is the number of fatalities on a per-vehicle-mile-driven basis (usually based on 100 million vehicle miles traveled) is a more accurate means of measuring highway safety.
  • The fatal accident rate, which is the number of fatal accidents on a per-vehicle-mile-driven basis (fatal accidents per 100 million vehicle miles traveled), is the most accurate means of measuring highway safety trends.

The reason fatality rates and fatal accident rates are a more accurate measure of highway safety trends is because they are based on the concept of "exposure." A motorist who drives 50,000 miles a year has 10 times the accident exposure risk than a driver who logs 5,000 miles in a year. Fatality rates measure the risk of being killed in an accident based on the number of miles traveled, or exposure.

Unfortunately, fatality figures are the numbers most often quoted by the media, insurance industry lobbying groups and even the National Highway Traffic Safety Administration (NHTSA). Organizations and individuals that quote raw fatality statistics claim "rates" are too complicated for the public to understand. Actually, fatality rates and fatal accident rates are very simple to determine and understand.

Hypothetical example:

Assume that in a given year, 20,000 motorists drive 20,000 miles each and fatal accidents kill eight people. The fatality rate, per 100 million vehicle miles traveled (the recognized standard for measuring fatalities), is two. The next year, the number of drivers doubles to 40,000, with each of them driving 20,000 miles per year. Everything else being equal, it makes sense that 16 people will die in accidents. However, the fatality rate remains identical: Two deaths per 100 million vehicle miles traveled. Although fatalities doubled, the highways were equally safe in each of those years.

In this hypothetical situation, the public's key source of information on highway safety -- the media -- will likely quote raw fatality numbers given by self-proclaimed "safety" organizations, with headlines such as, "Traffic Fatalities Double From 1995 to 1996!" Although accurate, the statement is completely misleading.

It also shows how comparing percentile changes of raw numbers, particularly when examining small populations (such as individual states), can be extremely misleading. For example, from 1995 to 1996, Missouri experienced an increase in fatalities from 1,109 to 1,149, a 3.6-percent increase. Yet highways were equally safe in both of those years, as is shown by the fatality rate, which remained the same, at 1.9.

Nationally, although fatalities increased by 90 from 1995 to 1996, from 41,817 to 41,907, the fatality rate actually declined. So, while the public saw headlines that read, "Fatalities increase . . .," the media unknowingly failed to also tell the public that the fatality rate dropped 2.2 percent, from 1.726 to 1.688. In reality, for the individual motorist, the highways had become safer!

Fatal Accident Rate

The objective of highway safety experts is to reduce accidents by determining their causes and eliminating or reducing those factors. An accident -- whether it involves a lone motorcyclist, a bus filled with passengers; or multiple vehicles -- is still a single accident.

Therefore, the most accurate measure of highway safety is the fatal accident rate. Like the fatality rate, it is determined by dividing the number of fatal accidents into the number of miles traveled. This formula eliminates the chance that short-term anomalies -- such as a rash of multi-vehicle or multi-passenger accidents in a certain state -- will cause fluctuations in the rate that are not related to the true cause of the accident or accidents.

For example, from January through August of 1997, the state of New Mexico experienced a slight increase in multi-fatality accidents involving vans. Compared to the same period in 1996, the number of fatalities increased from 309 to 326. However, the number of fatal accidents actually declined during that same period, from 269 in 1996 to 264 in 1997. Even if the vehicle miles traveled (VMT) in New Mexico did not increase from 1996 to 1997 (highly unlikely, as it has increased each year for decades), the fatality rate would have increased. The fatal accident rate, however, would have decreased.

Why per-mile rates are most accurate

Since the objective of those who are looking at highway accident data is to determine whether it is more or less safe to drive on the highway system, it is crucial to recognize that the exposure to the risk only exists when a driver is in a vehicle, on a road. Fatality rates based on population, the number of registered vehicles or the number of licensed drivers are virtually meaningless as analytical tools or measures of highway safety trends.

Also, cities, states and regions have vastly different characteristics, with varying demographics, vehicle use, road conditions, driving environments and driver training systems. Therefore, the per-person method of determining rates makes city-to-city or state-to-state statistical comparisons quite meaningless.

North Dakota and New Jersey are, in many respects, two very different states. These differences include population density, congestion, road conditions and topography. In 1995, the fatality rate (per 100 million vehicle miles traveled) was 1.3 in New Jersey, while it was 1.1 in North Dakota. Clearly, on a per-mile basis, it is more dangerous to drive in New Jersey because of factors such as congestion, poorer roads, topography and roadside obstacles.

However, when the fatality rate is based on the number of licensed drivers, it appears that North Dakota is a more dangerous place to drive (14.06 fatalities per 100,000 licensed drivers in New Jersey, versus 16.07 in North Dakota). The reason for this apparent inconsistency is because a typical licensed driver in North Dakota is not identical to a typical licensed driver in New Jersey. An average New Jersey driver logs fewer miles than an average North Dakota driver because everything in New Jersey is in much closer proximity. In North Dakota, a trip to the grocery store or post office may involve driving 20 miles, whereas in New Jersey it might be 20 blocks. Simply put, New Jersey drivers face less of an exposure to being in an accident. Because the per-licensed-driver rate does not account for this exposure, it is a meaningless statistic, especially when used to compare different geographic regions or states.

Likewise, when the rate is determined by using population as a denominator, the numbers show similar results (9.73 fatalities per 100,000 people in New Jersey, versus 11.54 fatalities per 100,000 people in North Dakota). Just as the "per-100,000-licensed-drivers" baseline is meaningless because it does not account for exposure, so is a rate based on population. The same explanation for why this rate is higher in North Dakota applies: the average North Dakota driver logs more miles than the average New Jersey driver, and therefore has a greater exposure.

VMT Figures Unreliable

Because estimates of the number of vehicle miles traveled each year are largely based on crude estimates, VMT data for a given year or a given state cannot be construed as highly accurate. Therefore, fatality and fatal accident rates that are based on VMT are not appropriate for detailed state-to-state or year-to-year comparisons. (Officials at the Department of Transportation, which collect VMT data, will confirm this.)

However, VMT figures do provide researchers with a reliable means of looking at long-term highway safety trends over the course of several years, and for the nation as a whole. This is because the effects of short-term fluctuations are insignificant when looking at long-term highway safety trends.

The long term trend for both the fatality rates and the fatal accident rates shows a continuous decline. The fatality rate has declined significantly for decades prior the passage of the 55 mph National Maximum Speed Limit (NMSL), declined during the 21-year NMSL era and has continued to decline since repeal of the NMSL in 1995.

Year Fatality Rate
(Per 100 MVM)
Fatal Accident Rate
(Per 100 MVM)
1921 24.1 NA
1930 15.1 NA
1940 10.9 NA
1950 7.2 NA
1960 5.1 NA
1970 4.7 NA
1980 3.3 3.0
1985 2.5 2.2
1990 2.1 1.9
1995 1.7 1.5

Source: National Highway Traffic Safety Administration

Making Sense of Highway Safety Data

When it comes to contentious issues, the setting of speed limits creates as much debate as gun control or abortion. Not only is it an issue that affects the vast majority of the public, but the debate has always centered on statistics -- particularly accident and fatality data. It would seem that the more data that is available on a given subject, the easier it would be for opposing groups to reach a consensus. Unfortunately, that is not the case with speed limits.

While there is absolutely no valid statistical evidence showing a correlation between speed limits and highway fatalities, various insurance industry lobbying groups and federal agencies continue to promote this false notion. Their strategy is to present and posture the data that most readily implies such a correlation exists.

In reality, many factors affect the number of people who are killed and injured on U.S. highways. The issue is far more complex than numbers placed on speed limit signs. This is especially true considering that current posted speed limits are often much lower than the speeds most motorists are driving. The myth that speed limits are a significant factor in accidents is without foundation. Most people drive at speeds that feel comfortable to them, regardless of the speed limit.

Traffic engineers have known for decades that the best and most effective speed limits reflect the speeds most motorists typically travel. Unfortunately, elected officials who are often charged with establishing speeds limits, generally ignore traffic engineers. Traffic engineers know that setting speed limits at the speeds drivers travel on a given road will improve traffic flow and will likely reduce accidents. They also understand that there are many factors involved in traffic accidents, the speed limit being least among them.

Numerous, Complex Factors

There are both long-term and short-term factors that can influence the number of highway fatalities (and the more important fatality rate).

Actions that have helped slow highway fatalities include safer highway designs, better signs, and the use of protective barriers. The number of multi-lane, limited access highways, which are far safer than other types of highways, continue to expand in mileage and use.

Americans also enjoy the continuing incremental improvements in the motor vehicle fleet. Automobiles are simply much safer, thanks to design improvements, better safety devices, and much improved handling capabilities. In the 1930s, one third of all automobile accidents were caused by equipment failure, most commonly tire blowouts and brake failures. In recent decades, estimates of the percent of accidents caused by equipment failure are as low as 2%.

Other long-term factors, such as changes in the demographic makeup of a given state or region, or the country as a whole, may have either a positive or negative effect on highway fatalities. For example, the U.S. population is aging. As "baby boomers" reach retirement age, they will have a significant impact on the U.S. highway fatality rate. While we may see a drop in fatalities, because older drivers log fewer miles, we might very well experience an increase in the fatality rate. That is because on a per-mile basis, elderly drivers have a higher than average fatality rate, similar to that of teen-age drivers.

* Short-term factors

Short-term factors that lead to fluctuations in accident numbers include inclement weather, or even more severe aberrations, such as war. Consumers and businesses alike cut back on discretionary travel during times of economic uncertainty, and the number of fatalities may subsequently decline. With fewer miles being driven, however, the fatality rate may very well stay the same, or perhaps even increase.

* Economy

Recessions and struggling economies are also notorious for their limiting effect on vehicle travel. Keep in mind that the type of travel motorists cut back on during these times is often the more risky, discretionary travel.

On the other hand, thriving economies -- regional and national -- always boost the number of miles traveled, particularly discretionary travel. Quite simply, more people drive more miles during good economic times. Compared to business travel, discretionary travel such as vacation travel usually includes multiple vehicle occupants, which means that more people are exposed to the risk of getting into an accident. Also, these drivers are often traveling on unfamiliar roads in unfamiliar terrain and often in unfamiliar weather conditions.

When there are more people driving more miles, fatalities will usually increase. The fatality rate and fatal accident rates, however, often remain the same or decrease during these times. Self-proclaimed automobile and highway "safety" organizations, as part of their campaign to reduce driving, claim holiday weekends are the most dangerous times of the year to be on the roads. In reality, holiday travel is often no more dangerous for the individual motorist than travel at other times of the year.

* Aberrations

Aberrations in fatality statistics are directly and indirectly caused by unexpected and typically non-repeating circumstances such as earthquakes, war or an unusually harsh winter. They may affect the entire country, but they are more often regional or state-specific, affecting highway safety in various ways.

Imagine if the United States finds itself in a squabble with the Arab nations and ends up facing an embargo on imported oil. Gasoline prices would skyrocket, and most states would see a severe cutback in discretionary travel. Drivers will make fewer trips, and the trips they make will only be as long as necessary. There will also be less discretionary travel. Many sections of the country will therefore see a dip in fatalities. However, with a corresponding decrease in miles traveled, fatal accident rates could conceivably remain the same. (Current VMT estimating procedures are biased against showing declining vehicle mileage travel. Also, they are unable to distinguish what type of travel -- business, commuting or vacation -- is being affected.)

Meanwhile, with the flow of oil being cut off from the Middle East, the state of Texas might suddenly find itself amid an economic boom as the country looks to it to help meet national petroleum needs. Texas drivers may log many more miles, and discretionary travel will increase. Fatalities will likely increase. Once again, however, with an increase in miles traveled, the fatality rate and fatal accident rate may not necessarily increase.

Causes of Highway Fatalities

When the focus is on the causes of accidents, this is where the fatality rate and the fatal accident rate part company. Factors which cause accidents have no relationship with the number of vehicles or passengers involved. This is why, when studying the causes of fatal accidents, the fatal accident rate is the best available measure of highway safety. It is not subject to fluctuations caused by the chance number of people involved in any given traffic accident.

Example: Assume that in two nearly identical rural counties which experience an average of one fatal accident per year, each has an accident in which a deer crossed a road, causing the driver to swerve and hit a tree. In county A, a lone motorcyclist was killed. In county B, seven passengers on a bus were killed. Assuming all drivers in both counties logged the same number of miles during the year (VMT), the fatality rate would be seven times higher in county B, but the fatal accident rate would be identical. In each county, there was one fatal accident with one common cause. If someone compared the two counties using the fatality rate, they could come to the conclusion that motorists are seven times more likely to die in a car-deer accident in county B. In reality, the odds would be identical, and the fatal accident rate would show that.

Suspicious and Arbitrary Categories

Unfortunately, American motorists have been misled about the various factors that lead to accidents. In any given accident, there may be many causal factors involved. Federal highway safety data will list an accident caused by a driver failing to yield as "speed-related" if the driver was exceeding the posted speed limit. It doesn't matter that the driver was exceeding the limit by only two miles per hour. It doesn't matter if most drivers on that road routinely exceeded the posted limit. In fact, accidents in which the drivers were traveling significantly below the posted speed limit are often recorded as "speed-related."

"Alcohol-related" is another arbitrary category. For example, the media often fail to explain to the public that NHTSA defines a fatal accident as "alcohol related" if either a driver or a nonoccupant, such as a pedestrian, had a blood alcohol concentration (BAC) of .01 grams per deciliter (g/dl) or greater. The BAC threshold for a DWI arrest in most states is .10, or 10 times the amount at which an accident qualifies for the "alcohol-related" category.

This means that if a pedestrian who had been drinking steps off a curb and is struck by a sober driver, the accident is listed as "alcohol related." The deception continues when the term "drunk driving" is substituted for "alcohol related," even though in many of these accidents, the driver or drivers were not suffering any meaningful level of impairment.

The most significant cause of accidents is not speeding. It is not drunk driving and it is certainly not the trendy "road rage." The number one cause of automobile accidents is -- unglamourous as it may be -- inattentive driving or driver error.

For decades the motoring public has been deceived into thinking that traffic law enforcement can significantly improve highway safety. This enforcement philosophy ignores the causes of most accidents and relies on the myth that drunk driving, speeding and aggressive driving are the most frequent causes. Billions of dollars and countless years of labor have been fruitlessly expended based on this premise, with no proven results.

Final Note: Putting Traffic Deaths Into Perspective

Sadly, traffic accidents are one of the leading causes of accidental death in the United States. However, put into the proper context of a population of 265 million people, most of whom travel daily on our highways, clearly the risk of death in a highway accident has been seriously exaggerated. Compare, for the same population, the 1996 deaths from heart disease (733,834), cancer (544,278) and strokes (160,431) against the 41,000 highway-related fatalities. Or, compare against another significant accidental cause of death such as drownings (3,900), and consider how much time Americans spend swimming versus driving or riding in a motor vehicle.

Putting traffic deaths into perspective and understanding how to interpret accident data are two crucial steps toward eliminating much of the unnecessary debate and confusion which often surrounds this issue.

When you see a police car on the side of the road, it should make you feel more safe.
So why doesn't it?

Across the United States, even the most careful, safe drivers on the road would probably admit to being nervous when they spot a police officer enforcing traffic laws. Instead of inspiring feelings of safety, our traffic laws are used to create fear. Can this ever change?





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