New concepts in design-speed application

The design-speed concept, as presently applied, does not preclude inconsistencies in highway alignment. The basic problem, particularly in the range of design speeds below 90 km/h (55 mph), is the tendency on the part of the driver to continually accelerate and decelerate. A secondary problem is the speed differential between automobilies and trucks. To overcome these weaknesses in current practice a new concept in the definition and application of design speed is presented. The overall object is to meet driver expectations and to comply with his or her inherent characteristics to achieve operational consistency and improve driving comfort and safety. The principle used in the updated design-speed approach is the 15-km/h (10-mph) rule, which during periods of free-flow conditions, entails three considerations: (a) reduction in design speed should be avoided if possible, but if it is required, it should be no more than 15 km/h (10 mph); (b) within a given design speed, potential automobile speeds along the highway normally should vary no more than 15 km/h (10 Mph); and (c) potential truck speeds generally should be no more than 15 km/h (10 mph) lower than automobile speeds on common lanes. The tool to accomplish these goals is a speed-profile technique. The potential automobile and truck speeds are plotted along the proposed highway improvement, taking into account the joint configuration of the horizontal and vertical alignments and the individual curvatures and gradients. The procedure is applicable to design of new facilities, but is even more useful for determining corrective measures to upgrade existing facilities. /Author/