1.3.2 The Annual Incremental Fatality Risk
There were 182.6 million licensed drivers in the United States in 1997 (NHTSA, 1998, Table 5), the most recent year for which this statistic is available. In 1998, there were 24,729 driver fatalities in the United States U.S. DOT, NHTSA, (Traffic Safety Facts 1998, 1999, Table 53), suggesting that the average annual fatality risk is 1.35 × 10-4. Based on the information outlined above, it is possible to calculate fatality risk for individuals wearing seatbelts (RBelts) and for individuals not wearing seatbelts. In 1998, NHTSA (1999) estimated that 69% of all drivers wear seat belts. Hence, the total number of fatalities of 24,729 is equal to NDrivers× (31%×RRNoBelts×RiskBelts + 69%×RBelts), where RRNoBelts (the fatality relative risk associated with not wearing a belt) is 1.41. This equation implies that RBelts is 1.20 × 10
-4 . The corresponding value for drivers who do not wear their belts is 1.69 × 10-4. Hence, the incremental annual fatality risk for drivers who do not wear seat belts is 4.93 × 10-5 (1.69 x 10-4 – 1.20 x 10-4.
1.4 Driving a Small Vehicle Instead of a Large Vehicle
NHTSA (1997b) estimated for various types of crashes the statistical change in the fatality rate associated with a 100-pound decrease in passenger car vehicle weight. The results appear in Table A-3.
Table A-3. Passenger Cars: Effect of 100-Pound Weight Reduction on All Fatalities. (Light Truck Weights Unchanged)
Crash Type Fatalities in 1993
Crashes Effect of 100-Pound Weight Reduction Net Fatality Change Principal rollover 1,754 +4.58% +80 Hit object 7,456 +1.12% +84 Hit ped/bike/motorcycle 4,206 -0.46% -19
Hit big truck 2,648 +1.40% +37
Hit another passenger car 5,025 -0.62% -31
Hit light truck 5,751 +2.63% +151
Total 26,840 +1.13% +302
Source: NHTSA (1997b), Table 6-7.
The values in Table A-3 must be adjusted to eliminate from consideration those fatalities that represent individuals other than the driver of the vehicle, who is the main focus of this section.
• This analysis assumes that the fatalities in the “hit ped/bike/motorcycle” category represent almost exclusively deaths among the pedestrians, bicyclists and motorcyclists involved in those crashes. This category is eliminated from consideration because it does not represent driver fatalities.
• This analysis assumes that one-half of the fatalities in “hit another passenger car” category represent fatalities among individuals in other vehicles. The baseline fatality rate in this category is therefore reduced by 50%.
• This analysis assumes that all of the fatalities in the “hit light truck” category represent fatalities among individuals in the passenger car. This assumption reflects the fact that light trucks are more aggressive in multiple vehicle collisions than are passenger
vehicles. Likewise, the analysis assumes that all fatalities in the “hit big truck” category represent fatalities among individuals in the passenger car.
• In 1998, 70% of all vehicle occupant fatalities represented driver fatalities (NHTSA, 1998, Table 53). Therefore, the analysis reduces baseline risk values by 30%.
In addition, the analysis must eliminate from the NHTSA calculations the decrease in fatality risks accruing to non-occupants. Specifically, decreasing vehicle weight leads to a decrease in fatalities among pedestrians, bicyclists, and motorcyclists involved in crashes, and among occupants in light trucks and other cars involved in crashes. These benefits are addressed as follows.
• The elimination of the “hit ped/bike/motorcycle” category from the calculations, discussed above, addresses fatality rate benefits accruing to pedestrians, bicyclists, and motorcyclists.
• The 0.62% decrease in fatalities in the “hit another car” category has been replaced with an increase of 1.12%. This adjustment reflects the assumption that for a vehicle
occupant, a 100-pound decrease in vehicle weight has the same impact on the fatality rate for multi-vehicle crashes as it has on the fatality rate for crashes in which the vehicle hits an immobile object.
• There is no information available to adjust the fatality rate change estimated by NHTSA for the “hit light truck” category. However, since light trucks are relatively aggressive vehicles compared to passenger cars, a relatively small proportion of the fatalities in these crashes represent occupants of the light truck. Hence, it is unlikely that the fatality rate change estimated by NHTSA has a substantial benefit component reflecting decreased fatalities among light truck drivers.
This analysis further adjusts the baseline fatality rate to reflect the decline in total motor vehicle fatalities since 1993. Specifically, the analysis reduces the assumed baseline fatality rate by 1%, the amount by which total motor vehicle fatalities decreased between 1993 and 1998. Table A-4 outlines these computations.
Table A-4. Passenger Cars: Effect of 100-Pound Weight Reduction on Driver Fatalities. (Light Truck Weights Unchanged)
Crash Type Total Fatalities in 1993 Crashes Total Fatalities in 1998 Crashes Fatalities Among Occupants of Primary Vehicle Driver Fatalities -- Primary Vehicle Effect of 100-Pound Weight Reduction Net Fatality Change Principal rollover 1,754 1,736 1,736 1,215 4.58% 56 Hit object 7,456 7,381 7,381 5,167 1.12% 58
Hit big truck 2,648 2,622 2,622 1,835 1.40% 26
Hit another passenger car 5,025 4,975 2,487 1,742 1.12% 20
Hit light truck 5,751 5,693 5,693 3,985 2.63% 105
Total 22,634 22,407 19,920 13,944 265
In 1997, there were 182.6 million licensed drivers in the United States (NHTSA, 1998, Table 5). Hence, a 100-pound decrease in car weight increases the annual fatality risk by 265 ÷ 182.6 million, or by 1.45 × 10-6. Assuming that risk is a linear function of car weight, driving a 2,000 pound car rather than a 3,000 pound car increases a driver’s fatality risk by 1.45 × 10-5. Since the baseline annual fatality risk for a driver is 1.35 × 10-4, the relative fatality risk is 1.11.14
14 The baseline risk is the annual number of driver fatalities (24,729) (NHTSA, 1998, Table 53) divided by the total number of licensed drivers (182.6 million). This quotient is 1.35 × 10-4. The annual fatality risk for a driver in a car