Citation

Moyer RA. Highw. Res. Board Proc. 1935; 14(Pt1): 123-130.

Copyright

(Copyright © 1935, National Research Council (U.S.A.), Highway Research Board)

DOI

unavailable

PMID

unavailable

Abstract

Further tests on ice and snow were made because study of accident statistics in Iowa and Connecticut showed that a large proportion of skidding accidents occur in the winter. It was found that reasonably safe coefficients of friction (0.3 to 0.5) can be obtained on ice if the chains are designed to provide contact between chain and ice at all times and if the load is great enough to make the chains cut into ice. High coefficients are necessary when applying brakes and when driving on curves. To provide the greatest safety under all ordinary road conditions the analysis indicates that brakes should be adjusted to obtain the maximum braking power on surfaces with coefficients of from 0.6 to 0.8 using brake power rates of 1.5 to 2.0 The frictional requirements on curves were determined from theoretical analysis of the forces acting on the vehicle when driving at constant speed, when accelerating and when braking. When driving at constant or accelerating speed the driving force has a component which opposes centrifugal force, but when braking the braking force supplements the centrifugal force. This explains why braking on a curve is os hazardous. A maximum useful coefficient of friction of 0.3 is recommended for use in the design of highway curves. As the result of this study the maximum superelevation recommended where icy road conditions are encountered is 0.1 ft. per ft., and the maximum curvatures recommended are: for main roads 5 deg., for secondary roads 10 deg. and for mountain roads 15 deg. the need for transition curves for high speeddriving is analyzed. Speed is the greatest problem in safe operation on curves because: the coefficients of friction decrease with increased speed; stopping distances, steering agles and centrifugal force increase as the square of the speed; required transition length increases as the cube of the speed and the shift toward the center formed by the path of the car increases as he sixth power of the speed.