NHTSA Report Number DOT HS 806 359February 1983

A Preliminary Evaluation of Two Braking Improvements for Passenger Cars - Dual Master Cylinders and Front Disc Brakes

Charles J. Kahane, Ph.D.


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:


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

Effectiveness of front disc brakes

Cost of braking improvements

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

Benefits of braking improvements

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



Dual master cylinders

Disc brakes

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