Traffic capacity types; rotary intersection sketch

Traffic capacity of a road

Capacity of a road is the maximum number of vehicles (or PCUs) that can pass a given point or section of a lane or road in one direction (or both directions on a two-lane road) during a given time period (normally one hour) under the prevailing roadway, traffic and control conditions (HCM definition; IRC:106 for urban roads).

Types of capacity:

1. Basic capacity: the maximum number of passenger cars that can pass in a lane under ideal (best possible) roadway and traffic conditions.
2. Possible capacity: the maximum number that can pass under the actual (prevailing) roadway and traffic conditions, i.e. after adjusting the basic capacity for lane width, shoulders, gradient, heavy vehicles, etc.
3. Practical capacity (design capacity): the maximum number that can pass without unreasonable delay or restriction to the drivers' freedom to manoeuvre, at the chosen level of service. It is about 75-80 percent of the possible capacity (design service volume at LOS C for urban roads in IRC:106, and at LOS B for rural roads in IRC:64).

The capacity depends on lane width, lateral clearance, gradient, the share of heavy vehicles, the number of lanes and traffic control. Recommended capacities and design service volumes are given in IRC:106-1990 (urban roads), IRC:64-1990 and IRC:SP:73 (rural roads) and in the Nepal Road Standard 2070.

Sketch of a rotary intersection with geometric elements

          Approach road
              |   |
              | | |   <- splitter island
      ________/   \________
     /   Entry curve (R1)   \
 ---+     ,--------.        +---
    |   /  Central  \  <-- weaving
 ---+  |   island    |       +---
    |   \   (Rc)    /   Exit curve
     \___`--------'____/
              |   |
          Approach road

 e1 = entry width    e2 = non-weaving width
 w  = weaving width  l  = weaving length
 (clockwise one-way circulation for left-hand traffic)

Geometric elements: central island radius $R_c$, entry radius $R_1$, exit radius $R_2$, entry width $e_1$, width of non-weaving section $e_2$, weaving width $w = (e_1+e_2)/2 + 3.5$ m, weaving length $l$, and splitter islands.