TABLE OF CONTENTS It is unlikely that the traffic engineer alone will be required to design major highways. Highway design is a separate, albeit a closely related, discipline. At most the traffic engineer will provide preliminary layouts of access roads and intersections for developments and, therefore, a good understanding of basic highway design techniques and standards is needed.
Selection of an appropriate design speed may be considered the starting point for any scheme. The DMRB Technical Standard TD 9/931 outlines the methods for selecting the link design speed.
In practice, the design speed for a particular route might be contained within a policy decision by the highway authority (HA). The engineer should consult the HA closely and, if no advice is forthcoming, suggest a method for determining a suitable design speed. In which case, the design speed should be selected by observation of the actual behaviour of the vehicles on the road in question.
Vehicle speeds are affected by many factors including speed limit, horizontal and vertical alignment, visibility, highway cross section, adjacent land use, spacing of junctions, accesses, pedestrian crossings and maintenance standards. The general condition and design of vehicles and driver ability, which changes over time can have a significant effect on vehicle speeds. It is usual to use an 85th percentile speed as the design speed (the speed below which 85% of drivers travel). The 85th percentile speed is determined from speed surveys using a radar speed meter or automatic traffic counters...
