Establish Analysis Lane Groups Each intersection approach is initially treated separately, and the results are later aggregated. Thus, each approach must be subdivided into logical groupings of traffic movements for analysis purposes.
The Highway Capacity Manual provides detailed guidelines on this process. Generally, the process consists of placing like movements into movement groups (left-turn, right-turn, and through movements would be identified) and then translating movement groups to lane groups based on the allowable movements from each lane. The only time these two group designations differ is when a shared lane (left-turn and through movements allowed from the same lane) is present on an approach with two or more lanes. When shared lanes are present on an approach with two or more lanes, the Highway Capacity Manual employs an iterative procedure to identify the expected distribution of each movement type in each lane, based on the principle that drivers will choose the lane that they perceive will minimize their travel time through the intersection (delay). This procedure is beyond the scope of this book and, as a result, the subsequent example problems and end-of-chapter problems will provide specific lane distributions of traffic as necessary. Consequently, it is only necessary to make reference to lane groups for the remainder of the chapter.
Based on the lane and traffic movement distribution on an approach, lane
groups can be readily determined. The following general guidelines are offered for establishing lane groups:
• If an exclusive turn lane (or lanes) is present, it should be treated as a separate lane group.
• Each shared lane on an approach should be treated as a separate lane group.
• Any remaining lanes, which would be exclusive through lanes, should be treated as a separate lane group.