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))