A horizontal curve on a two-lane highway (with 11-ft lanes) has a central angle of 33 degrees and is designed for 60 mi/h with a 8% superelevation. First, if the PI is at station 300 + 00, what is the station of the PT and how many feet have to be cleared from the inside lane’s lane-shoulder edge to provide adequate stopping sight distance? Second, if the normal component of centripetal force was considered, how much shorter would the radius of the curve be?
SOLUTION Note: Open boxes in equations “ ” are to be completed by the reader
This problem illustrates a number of principles in horizontal curve design, including the magnitude of imprecision introduced by ignoring the normal component of centripetal force (which is standard practice in highway design). To begin, the radius to the vehicle (Rv) for a 60-mi/h design speed and a superelevation of 8% can be determined With this, to determine R (the radius to the center of the highway), half of the lane width is added to Rv obtained from Table 3.5 to get to the centerline of the roadway so,
= + = × =1 lane-width .