Speed limit-related issues on gravel roads.

Speed limit-related issues on gravel roads.

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    Final report; Summer 2006-Fall 2008.
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    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.
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