Chiang Mai Journal of Science

Inattention to standards during the horizontal curve planning for forest roads causes to decrease driver’s safety and increase construction cost of forest roads and irreparable damage to environment. One of the most important factors in standards observation is correct selection of standard horizontal sight line offset on horizontal curve. Thus, the objective of this study is (a) to compare the standard horizontal sight line offset with existing status in forest roads and (b) to assess the results of the effects of incorrect selection of horizontal sight line offset on earth working area. In this study, 24 horizontal curves were selected on forest roads. Then, according to the calculated sight distance using its standard equation, the existing horizontal sight line offset was calculated after presenting on commence point of forest road curve and passage of laser line along line of sight. In order to achieve standard horizontal sight line offset in forest roads the standard equation was used. The Road eng software was used to compare the standard and existing area of earth working operation. Results of this study showed that low radius in horizontal curves had significant effect on horizontal sight line offset of forest road.  So that was found significant  inverse relationship between horizontal curve radius and horizontal sightline offset (R² = 0.87). For curves with radius of more than 24 meter it is no require to horizontal sight line offset. There is significant difference between the existing horizontal sight line offset in forest roads and standard (P<0.05). The existing horizontal sight line offset is more than that of standard. The rate of this difference is 1.6 meter in station number of 8 and on center on horizontal curve. The existing area of earth working operation is more than standard to provide horizontal sight line offset. There is significant difference between the standard area of excavation and existing area on horizontal curve to provide horizontal sight line offset (P<0.05). This difference causes to increase excavation operation in 7 m² in some picket on forest road. To minimize the chances for injuries in road accidents, the horizontal sightline offset should be carefully selected, not only to minimize the total road cost but also to reduce  the environmental  impact and to improve driver safety.