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Technical Paper

Car Size and Safety: a Review Focused on Identifying Causative Factors

In the last few years a number of additions to the technical literature on relationships between car size or mass and occupant risk of fatality or injury have appeared. This new information is reviewed, synthesized and used as the basis for additional calculations aimed at better identifying causal factors. It is concluded that if a car crashes head-on into a 12,000 kg truck, the car driver is 36% more likely to be killed in a 900 kg car than in an 1,800 kg car solely as a result of differing Newtonian kinematics. Five studies from two countries consistently support that when cars of similar mass crash head-on into each other, driver risk is inversely related to the common car mass. Size is the dominant causative factor in this relationship, and in the higher rollover risk in lighter cars. Mass and size are causal factors in single-car nonrollover crashes. Mass exercises a dominant causal effect on car driver risk in crashes between vehicles whose masses differ by more than about 10%.
Technical Paper

Car Size and Safety - Results From Analyzing U.S. Accident Data

The results of studies performed by the General Motors Research Laboratories to investigate the relationship between car size (indicated by car mass) and safety are presented. Most of the studies reviewed rely heavily on one data set that has uniquely useful features. This is the Fatal Accident Reporting System (FARS) data set maintained by the National Highway Traffic Safety Administration (NHTSA), which is part of the U.S. Department of Transportation. This file contains detailed information on every fatal traffic crash occurring in the United States since 1975. The studies find that given a single-car crash, the unbelted driver of a 900kg car is about 2.6 times as likely to be killed as is the unbelted driver of an 1,800 kg car. The relative disadvantage of the smaller car is essentially the same when the corresponding comparison is made for belted drivers.
Technical Paper

Gasoline Consumption in Urban Traffic

Fuel consumption and dynamical variable data have been collected as a car was driven normally with low speed urban traffic. It has been shown, using regression analysis, that the single traffic variable of the 16 studied that can best explain fuel consumption per unit distance is the average trip time per unit distance. The regression results are similar for each of four different methods of sampling the speed-time history of the vehicle in traffic. Such fuel consumption data may therefore be collected by the method which is operationally most convenient. The variable, average trip time per unit distance, explained more than 71% of the variance in fuel consumed per unit distance in these experiments. An interpretation of this result is given in terms of engine-vehicle dynamics and the characteristics of urban traffic.
Technical Paper

Fuel Used to Accelerate Vehicles from Rest to Cruising Speeds

This research was performed to experimentally determine how the amount of fuel used to accelerate an initially stationary vehicle to a constant cruising speed depends on acceleration level. Initially stationary vehicles were accelerated at different rates to a constant cruising speed on a test track, and the amounts of fuel used to travel a fixed distance from the starting point were measured. By subtracting the amount of fuel that would have been required to travel this same distance at the cruising speed from the measured amounts, estimates of the additional fuel used to accelerate from standstill to the cruising speed were obtained. These estimates were then examined as a function of acceleration level, which was characterized by the time taken to reach the cruising speed. Tests were conducted for cruising speeds of 48 km/h, 64 km/h and 80 km/h. Factors investigated included different vehicles (8 in all), engines (L-4, V-6, V-8), and fuels (gasoline and diesel).
Technical Paper

Drivers Involved in Crashes Killing Older Road Users

5, 032 people aged 70 or older were killed on US roads in 2005. Of these, 827 were drivers killed in single-vehicle crashes. The remaining 4, 205 were all killed in crashes involving at least one other driver. While a vast body of literature has focused on older drivers, this paper addresses the other drivers involved in the crashes that account for 84% of the deaths of road users 70 and over. The other drivers can be placed into three categories. 1. Drivers of vehicles involved in crashes in which pedestrians aged 70 or older are killed. 2. Drivers involved in two-vehicle crashes in which drivers aged 70 or older are killed. 3. Drivers of vehicles in which passengers aged 70 or older are killed, and drivers of vehicles involved in crashes with vehicles transporting such passengers. Analysis using data for 2000-2005 finds that 89% of pedestrian fatalities aged 70 or older occurred in crashes in which the driver was aged 69 or younger.
Technical Paper

How to Make a Car Lighter and Safer

About the most firmly established vehicle-safety effect is that the heavier the vehicle, the lower are the risks to its occupants. Empirically data show that the additional mass of a passenger reduces driver fatality risk by 7%. While occupants of heavier vehicles enjoy increased safety, there are two important negatives associated with heavier vehicles. First, they increase risk to occupants of other vehicles into which they crash. Second, they consume more fuel. The size, or length, of a vehicle also affects safety. All other factors, including mass, being equal, a larger vehicle reduces fatality risk to its occupants. But unlike mass, it also reduces risk to occupants in vehicles into which it crashes. A quantitative relationship expressing fatality risk as a function of the mass and size of both cars involved in a two-car crash was derived in Causal influence of car mass and size on driver fatality risk, Am J Pub Health. 91:1076-81;2001.
Technical Paper

Airbag Benefits, Airbag Costs

This paper estimates that currently there are 250 million frontal airbags in the United States, which cost their owners $54 billion. In 2003 about 1.7 million of these deployed, 19,000 in fatal crashes in which over 8,000 vehicle occupants were killed sitting in seats protected by airbags that deployed. To date over 40,000 occupants have been killed sitting in seats protected by airbags that deployed. The growth of airbags increases the need to revisit the question of their cost-effectiveness, and also provides the data to do this. The cost-benefit comparison presented here relies on airbag effectiveness estimates and injury cost estimates published since 2000. Even after the deployment of 10 million airbags, their effect on injury risk remains uncertain, and the results presented here are sensitive to the injury-effectiveness values assumed.
Technical Paper

Gender and Age Influence on Fatality Risk from the Same Physical Impact Determined using Two-Car Crashes

Studies using the double-pair-comparison method found that fatality risk from the same physical impact is (28 ± 3)% greater for females than for males, and increases with age after age 20 at compound annual rates of (2.52 ± 0.08)% for males and (2.16 ± 0.10)% for females. The purpose of the present study is to investigate fatality risk from the same physical impact versus gender and age using a different method and data distinct from those in the other studies. Female to male fatality risk was estimated using two-car crashes in which the gender of the two drivers differed. Fatality risk from the same impact is found to be (22 ± 9)% greater for females than for males, and to increase annually after age 20 by (2.86 ± 0.32)% for males and (2.66 ± 0.37)% for females. The relatively close quantitative agreement between the present and double-pair-comparison estimates increases confidence in the validity of double-pair-comparison methods and the present method.
Technical Paper

Urban Traffic, Fuel Economy and Emissions- Consistency of Various Measurements

A number of studies of the effect of traffic conditions on fuel consumption and emissions are reviewed. A model based on driving vehicles in traffic is described in which the fuel consumption of a vehicle in urban traffic is expressed as a simple function of trip speed. Data from a variety of sources, including additional field data, detailed computer simulation, the same vehicle tested on different fixed urban driving schedules, and small segments of the Federal Test Procedure (FTP) have been all shown to fit the model. A similar model of HC emissions as a simple function of trip speed is derived from analyzing small segments of FTP data. Data from a variety of sources, including published EPA relations, detailed computer simulation, and dynamometer replication of street data have been all shown to fit this model. No simple models were found for CO and NOx.
Technical Paper

Automobile Braking Energy, Acceleration and Speed in City Traffic

Speed-time data obtained in two large-scale studies by following randomly selected vehicles in a number of cities with an instrumented car are analyzed here by dividing the data into trip segments between successive stops and computing values of traffic variables for the individual trip segments. Results from the two studies are found to be in good agreement. The analysis focuses on variables previously shown to be related to fuel consumption, particularly on the relationship between energy used to accelerate the vehicle, energy dissipated in braking, and mean traffic speed. Braking and acceleration are found to play a major role in determining tractive energy requirements in low speed urban driving, since about half the energy supplied to the wheels is used to accelerate the vehicle, and about two-thirds of the resulting kinetic energy is dissipated in braking.
Technical Paper

Seat Belt Usage and Risk Taking in Driving Behavior

This study tested the hypothesis that seat belt usage is related to driver risk taking in car-following behavior. Individual vehicles on a Detroit area freeway were monitored to identify seat belt users and nonusers. Headways between successive vehicles in the traffic stream were also measured to provide a behavioral indicator of driver risk taking. Results showed that nonusers of seat belts tended to follow other vehicles closer than did users. Users were also less likely than nonusers to follow other vehicles at very short headways (one second or less). The implications of these findings for occupant safety in rear end collisions are discussed.
Technical Paper

Car Mass and Likelihood of Occupant Fatality

The purpose of this work is to estimate the relationship between car mass and the likelihood that a car will have an occupant fatality. Car occupant fatalities in the 1978 Fatal Accident Reporting System (FARS) data were divided into those killed in two car crashes and those killed in non-two car crashes. The relationship between car mass and the relative likelihood of an occupant fatality in non-two car crashes was determined by examining the number of fatalities in cars of a given mass driven by drivers of a given age divided by an estimate of the number of registered cars of the same mass having owners of the same age. This involves assuming that owners and drivers are the same people -- clearly an approximation. This and other assumptions were necessary because of data limitations.