The following excerpt was taken from the 1994 edition of AASHTO's A
Policy on Geometric Design of Highways and Streets (pp. 119, 121, 124).
The approximate braking distance of a vehicle on a level roadway may be determined
by the use of the standard formula:
d = V2/(254*f)
Where:
d = braking distance, m;
V = initial speed, km/h; and
f = coefficient of friction between tires and roadway.
In this formula for the braking distance the f factor is used as an overall or a single
value that is representative for the whole of the speed change. Measurements show that f
is not the same for all speeds. It decreases as the initial speed increases. It varies
considerably because of many physical elements such as air pressure of tires, composition
of tires, tire tread pattern and depth of tread, type and condition of the pavement
surface, and the presence of moisture, mud, snow, or ice. The braking distance also
depends on the braking system of the vehicle . . . .
. . . Because of the lower coefficients of friction on wet pavements as compared with
dry, the wet condition governs in determining the stopping distances for use in design.
The coefficients of friction used for design criteria should represent not only wet
pavements in good condition but also surfaces approaching the end of their useful lives .
. . .
(p. 124) Effects of Grade on Stopping
When a highway is on a grade, the standard formula for braking distance is the
following:
d = V2/(254*(f ± G))
in which G is the percent of the grade divided by 100, and the other terms are as
previously stated. The stopping distances on upgrades are shorter, those on downgrades are
longer.