For many years, the approach to reducing motor vehicle injury and death has been the "3-E" approach: engineering, education, and enforcement. Crash incidents have determined or dictated which roadways need improvements, where and what to enforce, and what public information and education should be conducted.

At the Montana Department of Transportation (MDT) reports of crashes with resulting injuries, fatalities and/or property damage are analyzed much as departments of transportation all over North America examine crash data. Also included in the analysis is consideration of Average Daily Traffic (ADT)volume. The results of the analysis are used to determine sections of roadways with high crash incidents where engineering "fixes" may reduce the numbers of crashes.

Examples of "fixes" ranged from erecting warning signs (such as deer crossing warning signs at locations of high deer-vehicle collisions), to installing traffic control devices (such as traffic lights with pedestrian crossing signals at locations with vehicle-pedestrian collisions), to redesigning road geometrics (such as changing edge-of-road slopes, redesigning intersections to include turning lanes or accommodations for larger vehicles such as semi-tractor-trailers, or adding additional traffic lanes). This same crash and ADT data might also be used to determine what enforcement and educational actions could impact on the crash problem.

According to Gerald Wilde’s Risk Homeostasis Theory (RHT), engineering or the other two Es fixes may "solve" the initial crash problem at a particular location, but other types of crashes may increase BECAUSE of the engineering, enforcement or education solution. My experience with the MDT projects I was assigned to provide evidence of risk homeostasis.

For example, an intersection on a major highway through a downtown urban area had a history of vehicle-to-vehicle crashes. The existing traffic control devices consisted of stop signs for traffic approaching on the side-street, while drivers on the 4-lane highway were assigned the right-of-way and were not required to stop. People attempting to access the highway often became impatient waiting for access and often attempted to enter the traffic flow when adequate space was not present. This action results in the vehicles with the right-of-way attempting to avoid a collision by swerving into another lane of traffic, or crashing into the people attempting to access this busy highway.

When a pedestrian was killed in attempting to cross the four lanes of the highway, the intersection was considered for the installation of a traffic light with pedestrian crossing lights. Shortly after the traffic lights were installed, this intersection had an increase of rear-end collisions, as those drivers who prepared to stop for the red traffic light were rear-ended by those who didn’t observe the changing of the traffic light from green to yellow to red.

Another section of highway presented a different problem. Interstate 90 near Butte, Montana had a 6-1/2% to 7% grade on the eastside of the continental divide, contributing to numerous semi-tractor-trailer crashes and fatalities at the various curves/switchbacks. Warning signs were installed in an attempt to alert truckers and other drivers of the dangerous curves and steep downgrades. Crashes with injuries and/or fatalities continued to occur. Two curve locations where "runaway" semi-tractor-trailers crashed into the Jersey guardrail in the median contributed the largest number of crashes.

The next engineering attempt to reduce crashes at these sites was to flattened east-bound downgrade curves; this "fix" only shifted the incidents of crashes to new locations. A runaway truck ramp was installed next, which seemed to help the semi-tractor-trailer drivers who suspected overheating brakes, but crashes continued to occur on these steep downgrades.

In both cases cited one kind of risk seems to take the place of another kind of risk and the total risk remained unchanged. It seems when drivers are aware of a dangerous or risky road, they tend to drive with a certain amount of caution and complain to authorities to "fix" the road. Often when roads are redesigned, reducing severity of curves and/or ditches are re-sloped to be more "forgiving", drivers perceive roads to be safer, so they decide to drive faster, thus increasing their risk of severity of crash due to higher rates of speed on the re-designed "safer" road. This kind of behavior has a name, risk homeostasis, and can be explained by Wilde’s RHT

The application of the 3-E approach (engineering, education, enforcement) to reducing the risk of motor vehicle injury and death, according to Wilde, is like attempting to dam one fork of a flowing river. The flow simply spills over into the others parts of the river basin.

If we "fix" all the roads to be as safe as possible (which is financially impossible), drive the safest vehicles possible, legislate and enforce safe driving practices, and educate motorist to follow safe practices motor vehicle crashes resulting in injury and death will continue. If we don't address the source of the water flow or risk taking, RHT predicts that people will continue to take risks leading to about the same crashes. We need to examine WHY people take risks.

In Target Risk, Wilde states "all decisions are risky decisions". Economists ask the question of costs vs. benefits. Traffic Safety Educators should ask a similar question. Let us examine WHY we take the risks we do and explore the potential benefits and the possible costs of risk taking.

Wilde states in Target Risk, "A variety of factors determine the extent of the accident risk that different people are willing to take. When the expected benefits of risky behavior are high and the expected costs are perceived as relatively low, the target level of risk will be high. The term "target" is meant to be synonymous with "preferred, desired, accepted, tolerated, and subjectively optimal". RHT states that the target level of accident risk is determined by four categories of motivating factors:


1) The expected advantages of comparatively risky behaviour alternatives: for instance, gaining time by speeding

2) The expected costs of comparatively risky behaviour alternatives: for instance, automobile repair expenses, insurance surcharges for being at fault in an accident.

3) The expected benefits of comparatively safe behaviour alternatives: for instance, an insurance discount for accident-free driving.

4) The expected costs of comparatively safe behaviour alternatives: for instance, using an uncomfortable seatbelt, being called a wimp by one’s peers.


"The higher the values in categories 1 and 4, the higher the target level of risk. The target level of risk will be lower as the values in categories 2 and 3 rise. Some of the motivating factors in all four categories are economic in nature; others are of a cultural, social or psychological kind.

With experienced drivers the motivating factors are usually so thoroughly internalized that most people, most of the time, are not consciously aware of them. Thus, the target level of risk should not be viewed as something that people arrive at by explicitly calculating probabilities of various possible outcomes and their respective positive or negative values."

"A person’s target level of traffic accident risk is defined as that level of subjective accident risk at which the difference between benefits and costs (including the perceived danger of accident) is believed to maximize. There may be cases in which risk is deliberately pursued, but most risks that people incur are rather more passively accepted as the inevitable consequences of their deliberate choice of action. Anybody who takes to the road knows that they might have an accident, either because of their own behavior, or because of the behavior of other road users that cannot be predicted, let alone controlled."

Risk Homeostasis Theory maintains that "in any activity, people accept a certain level of subjectively estimated risk to their health, safety, and other things they value, in exchange for the benefits they hope to receive from that activity (transportation, work, eating, drinking, drug use, recreation, romance, sports or whatever)."

Does believing in RHT mean throwing your hands in the air and doing nothing to address the costs of risk taking? Does it mean the abandonment of the three Es? Even if you do not believe in RHT can you afford to ignore the possibility that it maybe true? The answer to all these questions is No.

The engineering, enforcement and education communities should explore RHT and adjust their responses to both the cost and benefits of risk taking. As traffic safety educators, we must be aware of the risks and address the decisions that come with the driving task recognizing the reward of risk taking. We must prepare drivers for making the best decisions with regard to risk. We need to remember that "Education that brings no change is as effective as a parachute that opens on the first bounce".

So how can traffic safety educators avoid delivering education that works after the first crash? The answer is to recognize why people change their behavior and to ask: What will it take to get you to lower your target level of risk taking?

In Minnesota a different way of teaching decision making is being explored. This approach starts with these two related questions: What will it take to get you to change? How uncomfortable do you have to feel before you will take action to reduce your discomfort? The theory behind this approach is that people do not change unless they have reached their target level of discomfort. Sound familiar? It should because the approach is a response to RHT and the need to improve education’s response to risk taking.

To directly address sources of discomfort traffic safety education needs to provide learning opportunities that enhance knowledge, awareness of feelings, and the ability to think. Traffic safety education should result in changes that will result in an increased likelihood that it’s graduates will sense the discomfort (unacceptable risk) know how to respond to reduce their discomfort and have their parachutes open when they should.


Excerpts from Target Risk

Reviews and comments on Target Risk and the theory of risk homeostasis

Wilde, G.J.S. (1994) Target Risk: Dealing with the Danger of Death, Disease and Damage in Everyday Decisions PDE Publications, Toronto, Ontario Canada


C. Goebel Frahm, B.T., B.S., M.S. formerly of Traffic Design of the Montana Dept. of Transportation, and Minnesota Highway Safety Center at SCSU with John W. Palmer, Editor