Why does the fatality rate go up when the passenger car collides into the rear-end of a large-sized truck? The reason: A large truck has a large and solid cargo bed positioned higher than the height of the car, making the colliding car more liable to underrun below the cargo bed and causing the car's passenger compartment crash directly with the cargo bed as shown in Photo 1.   To prevent such underrunning, trucks were previously equipped with an underrun protector but these first-generation "structural" underrun protectors failed to reduce tragic underruns in which the car occupants were injured directly by the truck's cargo bed. Accordingly from June 1, 1992, it was made mandatory to equip large trucks with an "improved" underrun protector.

To look at the fatality/injury reducing effect of "improved" underrun protectors, large trucks of 8 tons GVW over are classified into "improved" and "structural" underrun protector types according to the dates of their new-vehicle registrations.   Conditions 1) through 7) below are provided to more accurately analyze the effect of underrun protectors. Since "improved" underrun protectors were initially applied to large trucks only, analysis is applied also to medium trucks over 3.5 tons and less than 7 tons GVW to confirm that there is no difference according to registration date. CONDITIONS 1) Only those rear-end collisions in which a passenger car collided into the rear-end of a truck are selected for analysis. 2) Since the installation of an "improved" underrun protector on new large trucks was made mandatory from June 1, 1992, the period     from six months before to six months after this date is considered a time of mixed registration of "improved"/"structural" types.     Trucks registered during this one-year period are therefore excluded from analysis. In other words, trucks registered prior to     December 1991 are assumed to be "structural" type, and trucks registered in January 1993 or later to be "improved" type. 3) Trucks registered in 1981 onward are analyzed, not including tractors. 4) The applicable passenger cars are standard-size sedans (sedans A to C, sporty & specialty sedans, and wagons in the ITARDA     vehicle classification), while one-box and tall recreational vehicles are excluded from analysis. 5) Only drivers, not passengers, are examined because there are differences in what components such as the steering wheel are     present in front of occupants depending on their seating positions. 6) The number of fatalities plus the number of the seriously injured are analyzed. 7) Rear-end collisions involving three or more vehicles are excluded from analysis. Data are taken for analysis from the Comprehensive Database on Traffic Accidents and cover the eight years from 1992 to 1999. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

The results of analysis are summarized in Table 1, according to truck gross weight, registration year, car driver's seatbelt wearing condition, and injury condition.

To examine the effect of "improved" underrun protectors, fatality/serious injury rates*3 are calculated for rear-end collisions up to 1991 and those in 1993 onward [Figure 7]. The bars represent average fatality/serious injury rates, and the thick lines indicate 95% probability ranges. Note 3: Fatality/serious injury rate = (Driver fatalities + serious injuries) / Total No. of drivers (i.e., dead + seriously injured + slightly injured + not injured) x 100   Taking the cases of large trucks 8 tons GVW over and the car drivers wearing the seatbelt, the fatality/serious injury rate proved to be 11.6% with a "structural" underrun protector and 7.6% with an "improved" underrun protector. When the drivers did not wear the seatbelt, the fatality/serious injury rate rose to 23.6% with a "structural" and to 14.7% with an "improved" underrun protector. Apparently, whether or not the driver has the seatbelt on, "improved" underrun protectors did result in a lowered fatality/serious injury rate.

Considering the average values and 95% probability ranges indicated in Figure 7, it can be judged that the fatality/serious injury rate significantly declined in 1993 onward as compared to the preceding years among large trucks of 8 tons over.   Although trucks over 3.5 tons and under 7 tons are yet to have an "improved" underrun protector, average values seem to indicate a higher fatality/serious injury rate in 1993 onward than in the preceding years. But 95% probability ranges suggest that this difference is not significant. Consequently it can be concluded from the lowered fatality/serious injury rate for trucks of 8 tons over that underrun protectors are indeed effective.

The above analysis has proved the positive effect of "improved" underrun protectors installed on large trucks in reducing injury to passenger car drivers. In this connection, however, medium and light trucks less than 8 tons GVW have been subject to the mandatory installation of a "structural" underrun protector since December 1, 1973. Consequently a shift from mandatory "structural" to mandatory "improved" underrun protectors is scheduled by the government.   Of the medium and light classes, an "improved" underrun protector already became mandatory to trucks 7 tons over and under 8 tons from October 1, 1997. In addition, an "improved" protector will be mandatory to ordinary trucks exceeding 3.5 tons (not including tractors) from September 1, 2005 and to light trucks exceeding 3.5 tons from September 1, 2007.

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Institute for Traffic Accident Research and Data Analysis (ITARDA)