The general terrain procedure for two-lane highways in the Highway Capacity Manual contains two flaws which could be significantly improved by changing the definition for when a vehicle is delayed. The first problem is that, even under the best roadway and traffic conditions, any two-way flow rate greater than 43 percent of capacity falls in levels of service D or E. Furthermore, levels of service D and E are too broad to provide definitive information about the flows within these levels. By changing the definition for when a vehicle is being delayed from a headway of 5 seconds, as given by the manual, to a headway of 3.5 to 4.0 seconds, more useful level-of-service categories result. The second problem is that the general terrain procedure does not yield results compatible with the specific grade procedure. Given otherwise identical traffic and roadway conditions, a two-lane highway will often be categorised as having a better level of service on a specific grade than on a level or rolling terrain segment. This is opposite of what one would expect. By reducing the delay definition to 3.5 or 4.0 Seconds, this inconsistency is not completely eliminated but is greatly reduced. Cases could be made for defining the delayed headway as any value from 2.0 to 6.0 seconds. For the purposes of the Highway Capacity Manual, a value between 3.5 and 4.0 seconds would provide more reasonable and consistent results.
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