Space Mean Speed
Calculate the space mean speed of two vehicles by the harmonic mean, v_s = 2 ÷ (1/v₁ + 1/v₂), from the individual speeds v₁ and v₂. The result, in the same unit as the speeds, is the harmonic mean — not the arithmetic — which is the correct way to compute the mean speed of a traffic stream when observing a road section (average over space). Space mean speed is always less than or equal to the time mean speed (the arithmetic mean observed at a point), because it gives more weight to slow vehicles, which spend more time in the section. It is the speed used in the fundamental equation q = k·v. Enter the two speeds.
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Velocidade média espacial
Existe uma sutileza importante em 'velocidade média' no tráfego: o resultado depende de como se mede. Se você fica parado num ponto e anota a velocidade de cada veículo que passa, a média aritmética dessas leituras é a velocidade média temporal (time mean speed). Mas se você tira uma 'fotografia' de um trecho de via e calcula a média das velocidades dos veículos ali presentes, obtém a velocidade média espacial (space mean speed) — e ela é a média harmônica, não a aritmética: para dois veículos, v_e = 2 ÷ (1/v₁ + 1/v₂). Por que harmônica? Porque um veículo lento passa mais tempo dentro do trecho, então está super-representado numa fotografia espacial — a média harmônica dá automaticamente mais peso a ele. Uma consequência matemática é que a velocidade média espacial é sempre ≤ à temporal (são iguais só quando todos vão à mesma velocidade). Essa distinção não é mera curiosidade: a equação fundamental do tráfego q = k · v só é válida com a velocidade média espacial. Usar a temporal por engano introduz erro nos cálculos de fluxo e densidade. Por isso radares e estudos de velocidade precisam saber qual média estão produzindo. Informe as duas velocidades.
Related Tools
Greenshields Speed
Calculate the speed of a traffic stream by the linear Greenshields model, v = v_f·(1 − k ÷ k_j), from the free-flow speed v_f, the current density k and the jam density k_j (vehicles/km). The result, in the unit of v_f, shows speed falls linearly with density: on an empty road (k = 0), vehicles travel at free-flow speed; as density rises, speed decreases, reaching zero at total jam (k = k_j). It is the most classic macroscopic traffic flow model, the basis of the parabolic flow-density relationship. Enter the free-flow speed, the current density and the jam density.
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.
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.