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.
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Number of lanes required
One of the first decisions in the geometric design of a highway or urban street is how many lanes it needs. The basic calculation starts from demand and capacity: N = V ÷ C_lane, the design traffic volume V (the demand forecast for the design horizon, usually the design hourly volume derived from average daily traffic) divided by the capacity of one lane C_lane. The result is the minimum lane count for the road to operate within capacity - in practice, though, two cautions apply. First, always round up to the next whole number (3.2 lanes means 4 lanes, since half a lane does not exist). Second, engineers design for a degree of saturation below 1 (a design V/C of 0.8-0.9, say), which in practice means using a reduced lane capacity or an inflated design volume, so the road never runs at the limit and keeps slack for growth and disturbances. Lane capacity itself depends on many factors: the road type (an uninterrupted-flow freeway carries ~2000-2300 veh/h per lane; a signalised urban street far less, since green is only a fraction of the cycle), design speed, lane and shoulder widths, and traffic composition. That lane count, together with lane, shoulder and median widths, defines the cross section of the road. Enter the traffic volume and the capacity per lane.
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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.
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.
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.
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.
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.
Timber Design Strength (kmod, NBR 7190)
Computes the timber design strength per the Brazilian NBR 7190, f_d = k_mod1 · k_mod2 · k_mod3 · f_k / γ_w, where the three modification factors correct the characteristic strength for load duration, service moisture class and timber grade, and γ_w is the material partial safety factor. Timber is the only common structural material whose strength falls with the DURATION of the applied load, and that is what k_mod1 encodes: it is 1.10 for instantaneous action and only 0.60 for permanent load, so the same member is worth nearly twice as much under impact as under self weight. In the most common design combination — long-duration action (0.70), moisture class 1 or 2 (1.00), first-grade sawn timber (1.00) and compression parallel to the grain with γ_wc = 1.4 — the factors cancel such that the design strength comes out exactly half the characteristic value, a shortcut worth memorising to sanity-check any result. Enter the three modification factors, the characteristic strength and the partial safety factor.
The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.