| NHTSA Report Number DOT HS 806 359 | February 1983 | A Preliminary Evaluation of Two Braking Improvements for Passenger Cars - Dual Master Cylinders and Front Disc Brakes Charles J. Kahane, Ph.D.
Abstract Dual master cylinders were installed in passenger cars in order to provide dual braking systems which is a requirement of Federal Motor Vehicle Safety Standard 105. Front disc brakes were installed to improve a car's handling capability during braking and to enhance resistance to braking losses due to fade or water exposure: they meet the Standard 105 requirements on fade and water resistance more readily than drum brakes. The objective of this Agency staff evaluation is to determine how many fatalities, injuries and damages are prevented by dual master cylinders and front disc brakes and to measure the actual cost of the devices. The evaluation is based on statistical analyses of North Carolina, Texas and Fatal Accident Reporting System data, a review of Indiana in-depth accident analyses, and manufacturing and repair cost analyses for production brake assemblies. It was found that: -
- Dual master cylinders annually prevent 40,000 accidents that would have resulted in 260 fatalities, 24,000 injuries and $132 million In property, damage. They add $17 to the lifetime cost of owning and operating a car (in 1982 dollars).
- Front disc brakes annually prevent 10,000 accidents that would have resulted in 64 fatalities, 5,700 injuries and $32 million in property damage. They add $21 to the lifetime cost of a car.
Summary Two of the most notable changes in the braking systems of domestic passenger cars during the 1960's and 1970's were the installation of dual master cylinders and the replacement of front drum brakes by disc brakes. Dual master cylinders are the chief component of a split or dual braking system. Without dual brakes, a failure in the hydraulic system can lead to catastrophic loss of braking power. With dual brakes, should one of the systems fail, the driver can still stop with the other. Disc brakes give the driver a better "feel" of the car's braking power because they have a more linear relationship between brake pedal pressure and vehicle deceleration than do drum brakes. In addition to improving a car's handling capabilities, they have potential safety benefits such as alleviating side- to-side brake imbalance due to improper maintenance, enhancing resistance to temporary braking power losses due to fade or exposure to water, and helping to prevent premature lockup of the front wheels during heavy brake applications. Federal Motor Vehicle Safety Standard 105 regulates the hydraulic brake systems of passenger cars (and certain other vehicles). There were two versions of Standard 105: 105-68, which became effective on January 1, 1968 and 105-75 which was effective January 1, 1976. Both versions consist primarily of a series of stopping tests simulating normal, adverse and emergency braking conditions. They also specify that cars shall have a dual braking system. Executive Order 12291 (February 1981) requires agencies to evaluate their existing major regulations, including any rule whose annual effect on the economy is $100 million or more. The objectives of an evaluation are to determine the actual benefits - lives saved, injuries prevented, damages avoided - and costs of safety equipment installed in production vehicles in response to a standard and to assess cost-effectiveness. This preliminary evaluation of passenger car braking improvements dots not cover all aspects of Standard 105 but is limited to dual master cylinders and front disc brakes. Dual master cylinders are clearly a safety device. They satisfy Standard 105's requirement for a dual braking svstem and were installed at least one year before its effective date. Disc brakes, on the other hand, are not required by Standard 105 and were not necessarily Installed for safety reasons alone. On the other hand, disc brakes were the most noticeable braking change of the late 1960's and earlv 1970's and made it considerably easier for cars to pass some of the specific stopping tests (fade and water recovery) of Standard 105-75. The accident reduction benefits for dual master cylinders and disc brakes were initially surveyed by reviewing in-depth accident analyses form the Indiana Tri-Level Study of the Causes of Accidents. Then, effectiveness estimates were obtained by statistically analyzing accident data from the North Carolina and Texas State files and the Fatal Accident Roporting System. Costs were estimated by analyzing braking system components of a representative sample of cars and by obtaining data on repair frequencies and costs. The most important conclusions of this preliminary evaluation are that dual master cylinders are a cost-effective safety device, saving 200-300 lives each year, preventing thousands of injuries and significantly reducing property damage in crashes. The conclusions on dual master cylinders can be drawn firmly because of the high level of consistency between the statistical analyses of three accident data files, the in-depth accident analyses and engineering intuition. Disc brakes, as stated above, are not required by Standard 105 and are not exclusively a safety device. Nevertheless, the evaluation indicates that disc brakes have significant safety benefits, although these are only about one-fourth as large as the benefits for dual master cylinders. The specific estimate of disc brake effectiveness is made with less certainty than for dual master cylinders, but at least it can be said that disc brakes are not harmful and in all likelihood beneficial, on the one hand, and do not have very large safety benefits, on the other. The principal findings and conclusions of the study are the following: Principal Findings Effectiveness of dual master cylinders -
- The fleetwide installation of dual master cylinders eliminated 40,000 reported accidents per year, which is 0.7 percent of all accidents involving passenger cars (confidence bounds: 0.58 - 0.82 percent). The accidents eliminated were those in which brake defects had been a contributing factor to the crash.
- Effectiveness was approximately the same in property, damage, injury and fatal crashes.
Effectiveness of front disc brakes -
- The fleetwide introduction of front disc brakes eliminated 10,000 reported accidents per year, which is 0.17 percent of all accidents involving passenger cars (confidence bounds: 0.10 - 0.24 percent). The accidents eliminated were those in which brake defects had been a contributing factor to the crash.
- Effectiveness was approximately the same in property damage, injury and fatal crashes.
- Effectiveness was just as great on dry roads in flat regions as on wet roads (possible water exposure conditions) or in hilly regions (possible fade conditions).
- In two-car front-to-rear collisions, disc brakes were not found to have anv effect on the likelihood that a car is the striking vehicle. In other words, disc brakes did not lead to a reduction of these types of accidents.
Cost of braking improvements -
- Tie costs per car (in 1982 dollars) for dual master cylinders and front disc brakes are the following:
| Dual Master Cylinders | Front Disc Brakes | | Initial purchase price increase | $ 9.50 | $ 2.90 | | Lifetime fuel consumption due to weight increase | 2.25 | 5.21 | | Lifetime repair cost increase | 5.20 | 12.97 | | TOTAL COST PER CAR | $16.95 | $21.08 | -
- The annual costs of the improvements in the United States (based on 10 million cars sold) are $170 million for dual master cylinders and $210 million for front disc brakes.
Benefits of braking improvements -
- The annual benefits, when all cars on the road in the United States have dual master cylinders and front disc brakes, will be:
| Reduction of | Dual Master Cylinders | Front Disc Brakes | | Best Estimate | Confidence Bounds | Best Estimate | Confidence Bounds | | Fatalities | 260 | 220-310 | 64 | 38 - 90 | | Nonfatal hospitalizations | 2,500 | 2,100 -3,000 | 610 | 360 - 860 | | Injuries (any type) | 24,000 | 19,000 - 28,000 | 5,700 | 3,400 - 8,100 | | Police-reported accidents | 40,000 | 33,000 - 47,000 | 9,800 | 5,800 - 13,800 | | Property Damage | $132M | $110 - 155M | $32M | $19 - 45M | Cost-effectiveness -
- An "Equivalent Fatality Unit" corresponds to I fatality or 16.9 nonfatal hospitalizations. Dual master cylinders eliminate 2.4 Equivalent Fatality Units per million dollars of cost (confidence bounds: 2.0 - 2.9).
- Front disc brakes eliminate 0.5 Equivalent Fatality Units per million dollars of cost (confidence bounds: 0.3 - 0.7).
Conclusions Dual master cylinders -
- Dual master cylinders have accomplished their objective of significantly reducing accidents due to brake failure.
- Dual master cylinders are a cost-effective safety device.
Disc brakes -
- Front disc brakes appear to have been effective in reducing accidents due to brake failure.
- Front disc brakes do not significantly reduce the numbers of accidents due to brake fade or exposure to water, relative to drum brakes of the late 1960's and early 1970's.
- Disc brakes do not appear to have had a significant effect in accidents that did not involve brake defects. The better "feel" and handling qualities of disc brakes did not result in a measurable safety payoff.
- It is tentatively concluded that the primary benefit of disc brakes is a reduction in accidents due to severe side-to-side brake imbalance.
- Disc brakes increase the cost of owning and operating a car primarily because their repair and maintenance costs are higher than for drum brakes.
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