Highway Lane Capacity at a Target v/c Ratio
Highway Lane Capacity at a Target v/c Ratio
A lane’s theoretical capacity is rarely the volume it should actually be designed to carry — operating right at capacity means any small disturbance collapses the flow into stop-and-go congestion. Instead, designers target a volume-to-capacity (v/c) ratio comfortably below 1.0 to keep a buffer for platooning, weaving, and incidents.This calculation finds the actual planning volume a lane can carry once that target ratio is applied to its base capacity, which is what feeds directly into lane-count and level-of-service decisions on a corridor study.
The sustainable volume is V = c·(v/c), the base capacity times the target volume-to-capacity ratio. where c is the base lane capacity and v/c is the target ratio of volume to capacity.
Inputs
c_cap := 2000 hr^-1=0.56 Hz
ratio_vc := 0.85=0.85
Scaling the theoretical capacity down by the target ratio gives the planning volume the lane should be designed around.
V_los := c_cap·ratio_vc=0.47 Hz
Results
A result near 1700 vehicles per hour per lane is a realistic planning volume for a freeway lane held at v/c = 0.85, which generally corresponds to level of service D. Pushing the target ratio closer to 1.0 raises the planning volume but leaves little margin before breakdown, so most agencies deliberately design for v/c around 0.8–0.9 rather than 1.0.
Assumptions & limitations
- Base capacity assumed for ideal conditions (no adjustment for grade, trucks, or lane width).
- Uninterrupted flow facility (freeway/multilane highway, not a signalized street).
- Target v/c ratio is a policy choice tied to desired level of service.