Level of Service (LOS) Calculator

Level of Service (LOS) is the Highway Capacity Manual's letter-grade system -- A through F -- for describing how a freeway or multilane roadway segment actually operates under a given traffic load, distinct from the raw hourly volume itself. This calculator converts your reported directional traffic volume into an adjusted flow rate by dividing out the Peak Hour Factor (which spreads a lumpy 15-minute peak across a full hour) and a heavy- vehicle adjustment factor (which converts trucks and buses into an equivalent number of passenger cars, since a truck occupies more effective capacity than a car at the same speed). That adjusted flow rate, combined with a speed-reduction curve that slows traffic as the volume-to-capacity ratio climbs, produces an estimated density in passenger cars per mile per lane -- the actual variable the HCM grades on. Total capacity is a separate figure driven only by free-flow speed and lane count: it does not know or care about the Peak Hour Factor, which only reshapes how a given volume is spread across time, not how many vehicles the roadway can physically process.

How much the heavy-vehicle percentage matters is scaled by Terrain, but it is never zero: this calculator uses HCM's standard general-terrain-segment passenger-car equivalents for trucks (ET) -- 1.5 on Level terrain (the default), 2.5 on Rolling terrain, and 4.5 on Mountainous terrain -- so even on flat ground each truck or bus consumes 50% more effective capacity than a passenger car, and Heavy Vehicle % always pushes adjusted flow up somewhat. That effect grows sharply as terrain gets steeper: the same heavy-vehicle percentage pushes adjusted flow up far more on Mountainous terrain than on Level terrain, because a truck struggling up a grade occupies proportionally more roadway capacity than one on flat ground. This is a planning-level HCM approximation, not a substitute for a full traffic study with field-measured speeds and calibrated capacity values for a specific facility.