Volume/Capacity Ratio (V/C)
Calculate the volume/capacity ratio (degree of saturation), X = V ÷ C, dividing the traffic volume V by the capacity C of the road or intersection. The dimensionless result measures the road's utilization: X near 0 indicates a free road; X = 1 means the road operating exactly at capacity; X > 1 indicates demand above capacity, with growing queues and congestion. The V/C ratio is the main indicator to classify the level of service (LOS A to F) and identify bottlenecks. Values above 0.85–0.90 already indicate near-saturation operation. Enter the volume and the capacity.
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Volume/capacity ratio (V/C)
The volume/capacity ratio (V/C), also called the degree of saturation (X), is the indicator most used to diagnose the performance of a road or an intersection: X = V ÷ C, the traffic volume demanded divided by the capacity available. The result, dimensionless, tells an immediate story about the state of the facility. X close to 0: a practically empty road, free flow, maximum speed. X around 0.5–0.7: stable traffic, already noticeable, with some loss of speed and of freedom to manoeuvre. X = 0.85–0.90: the road approaches saturation — small disturbances (a braking manoeuvre, a turning movement) start to generate waves of congestion, and operation turns unstable. X = 1.0: the road operates exactly at capacity, the point of maximum throughput and equally of maximum fragility — any rise in demand or any perturbation brings it down. X > 1.0: demand exceeds the capacity; a queue forms and grows continuously for as long as the excess demand lasts (congestion is not a state but a cumulative process), and the road shifts into forced flow, at very low speeds. The V/C ratio is the basis for classifying the level of service (LOS, on a scale from A to F, from free flow to breakdown) and for spotting the bottlenecks of a network — the points with the highest V/C are where investment belongs. Enter the volume and the capacity.
Related Tools
Saturation Flow
Calculate the saturation flow of a signalized approach, S = S₀ × N, multiplying the base saturation flow per lane S₀ (vehicles/h per lane, typically ~1800–1900) by the number of lanes N. The result, in vehicles/h, is the maximum rate of vehicles that can cross the stop line if the signal stayed green continuously and a queue existed — the queue discharge rate during green. It is a central parameter in signal design and intersection capacity, adjusted by lane width, grade, turning and parking factors. Enter the base saturation flow per lane and the number of lanes.
Average Headway
Calculate the average headway (time interval between successive vehicles), h = 3600 ÷ q, dividing 3600 seconds by the flow rate q (vehicles/h). The result, in seconds, is the average time between two consecutive vehicles passing a point. Headway is the inverse of flow: the higher the traffic volume, the shorter the intervals. It is a central concept of traffic flow theory, used in signal design, capacity analysis and car-following models. The smallest safe headway defines the maximum capacity of a lane. Enter the flow rate.
Equivalent Flow (PCE)
Calculate the equivalent flow in passenger car equivalents (PCE), q = Q_cars + Q_heavy × E, adding the car flow to the heavy-vehicle flow multiplied by the equivalence factor E (how many passenger cars each truck or bus equals in road occupancy — typically 1.5 to 3.0). The result, in PCE/h, converts a mixed traffic stream into an equivalent homogeneous one, allowing volumes to be compared and the capacity of roads with different traffic compositions to be computed. Heavy vehicles occupy more space and accelerate more slowly, especially on grades. Enter the car flow, the heavy-vehicle flow and the equivalence factor.
Average Vehicle Spacing
Calculate the average vehicle spacing, s = 1000 ÷ k, dividing 1000 metres by the traffic density k (vehicles/km). The result, in metres, is the average distance between the fronts of two consecutive vehicles in a traffic stream. Spacing is the inverse of density: congested roads have high density and small spacing; free-flowing roads have low density and large spacing. It is the spatial analogue of headway (which is temporal) and relates to speed by s = v·h. The smallest spacing, at jam density, equals the vehicle length plus the minimum gap. Enter the traffic density.
Number of Lanes Required
Calculate the number of lanes required on a road, N = V ÷ C_lane, dividing the design traffic volume V by the capacity of one lane C_lane (vehicles/h per lane). The result is the minimum number of lanes to serve the demand within capacity; in practice, always round up to the next integer. It is a basic sizing calculation in the geometric design of highways and urban roads, defining the cross-section from the predicted volume and the per-lane capacity (which depends on speed, road type and traffic conditions). Enter the traffic volume and the per-lane capacity.
Peak Flow Rate (PHF)
Calculate the peak flow rate of a roadway, q = V ÷ PHF, dividing the hourly volume V (vehicles/h) by the peak hour factor PHF (between 0 and 1, the ratio of the hour's volume to four times the busiest 15-minute volume). The result, in vehicles/h, is the equivalent flow rate of the busiest 15-minute period — always greater than or equal to the hourly volume, since traffic does not arrive uniformly. It is the design flow used in capacity and level-of-service analysis by the HCM, since sizing by the hourly average would underestimate the peaks. Enter the hourly volume and the peak hour factor.
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