Question & Answer Forum

 
Accident Fatality Rates (Posted 4/20/2007 8:34:10 PM)
The two most common denominators for accident fatality rates are "100,000 population" and "100MVMT". Looking for input on which measure is more appropriate.
Re: Accident Fatality Rates (Posted 8/1/2007 12:36:47 PM)
The answer to this question is "it depends". The population base is most commonly used in other parts of the world since VMT is so inconsistently gathered across countries. So for international purposes the population base is the more accurate and provides a more reliable base than VMT.

Within the U.S. we traditionally use VMT. It gives a sense of crashes taking the amount of traffic or exposure to other vehicles into account. The theory is that the more you are exposed to traffic the more frequently you are have risk of a crash. However, VMT can also hide the nature of the crash problem. If the volume of traffic increases the crash rate may go down but the number of crashes may remain the same. This is also true of a population base if population goes up and the number of crashes stays constant. For these reasons more and more safety professionals are using actual numbers of crashes, fatals, injuries, etc. along with one of the rate measures. Using both a rate and a real number and tracking both gives you a clearer picture of the crash situation. Occasionally you will see other measures used to calculate rate, e.g. number of vehicles. Again, for specific purposes this might be useful.

As you see I've not said whether VMT or population is more appropriate. The most comprehensive approach is to track both plus actual numbers of crashes.
Re: Accident Fatality Rates (Posted 10/31/2007 11:31:12 AM)
This is a very good summary. Rates can be a good indicator, but should not be the only indicator, nor should they be relied upon too heavily. While the per population rate might be good to compare nations and possibly states, it is not a good indicator within a state, since a small town could be at the junction of major roadways.

ADT rates provide a better indicator, but they also have a downside. Effectively they remove the largest cause of crashes, i.e., the presence of vehicles. Obviously, the more vehicles present at a given location, the more crashes can be expected. It should also be obvious that if we wish to reduce the maximum number of crashes (save the maximum number of lives), we are going to have to go to where the vehicles are (all other things being equal).

ADT-based rates tend to take us away from where the vehicles are to locations with a lower number of crashes occur but where the low volume of vehicles drives the rate up. No doubt, this will uncover some remote locations that are quite bad (death traps) that really need to be addressed. However, depending on rates alone will move resources away from the major problem areas, i.e., where the maximum number of vehicles are. Thus, it will not produce an optimal allocation of resources (e.g., maximum reduction in fatalities for the funds expended).

Another anomaly in rates is a diminishing return that occurs when roadways eventually get stuffed with so many vehicles that it is difficult to have a severe crash. Fatalities are rare when roads become parking lots, which is typical of many of our large urban areas. So, it is quite important to take into account the severity of the crashes that occur as well as the frequency. A good frequency metric is injury and fatal crashes, or, some feel better with severe injury and fatal crashes. Severe injury crashes predict future fatal crashes as well, if not better, than fatal crashes alone. The reason is that there are more of them to work with, and quite often it is just a matter of inches or the use of restraints that kept the crash from being fatal.

In summary, you are right on target with your "multiple rate and frequency approach." There is no single silver bullet.

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