In the United States, there are nearly 1.6 million miles of unpaved roads. Total length of unpaved roads in Kansas is

about 98,000 miles, of which about 78,000 miles are gravel roads. Most of the gravel roads are not typically posted

with speed limit signs but rather are regulated with a 55 mph blanket speed limit established by Kansas statutes.

Surface conditions of gravel roads are likely to change with time, space, and quality of maintenance work, making

it even more necessary to have proper control of traffic speeds on these roads. Normally used speed regulations

and rules for freeways or other types of paved roadways might not be appropriate for gravel roads, especially for

those local thoroughfares which usually carry very low traffic in rural areas. An extensive literature search revealed

no specific rules or references to provide guidelines on setting speed limits on gravel roads. Therefore, an effort was

made in this study to evaluate the effects of currently posted lower speed limits in some Kansas counties based on

traffic characteristics and safety on gravel roads, with the intention of providing proper guidelines for setting speed

limits on gravel roads in Kansas.

Speed analysis on a number of gravel roads where the statutory-imposed, frequently unposted speed limit of 55 mph

was utilized indicated that they are functioning at a reasonably acceptable level in terms of actual speeds. In order

to evaluate whether there were differences in traffic speeds between two counties or groups which have different

speed limit settings on gravel roads, a t-test was used. The analysis found no significant difference between mean

speeds in two counties, one of which has a 35 mph posted speed limit on gravel roads while the other did not post

any speed limits. Moreover, mean speed on sections with a 35 mph posted speed was a little higher than on gravel

roads without any speed limits. Linear models to predict 85th-percentile speed and mean speed on gravel roads were

developed based on speed data. Both models indicated that traffic speeds are not significantly affected by the speed

limit, but are related with 90% confidence to road width, surface classification, and percentage of large vehicles in

traffic. Chi-square tests were conducted with crash data, and the results indicated that the posted 35 mph speed limit

on gravel roads had not resulted in either smaller total number of crashes or decreased proportion of severe crashes,

compared to gravel roads where no speed limits were posted. Logistic regression models were also developed on

four levels of crash severity, which indicated that gravel roads with higher speed limits are likely to experience a

higher probability of injury crashes. However, special sections such as curves and bridges were also included in the

dataset considered in this analysis, making it impossible to make a direct comparison with the other sections.