Speed Hump Design Calculator

A speed cushion is really a set of narrower raised sections with gaps between them, sized so a standard car's wheel track still crosses a raised portion (getting the same slowing effect as a full-width hump) while a fire engine or ambulance's wider wheelbase can straddle the gaps and cross with less speed reduction. That tradeoff is deliberate: it's the standard way engineers add vertical traffic calming to a street that emergency services also depend on for fast response.

A speed table's flat top and longer ramps produce a gentler vertical acceleration on the vehicle for the same crossing speed compared to a shorter, steeper speed hump, which is why tables are typically designed for a somewhat higher target crossing speed. The tradeoff is a longer device footprint and, generally, less aggressive speed reduction than a hump sized for a lower target speed.

Gravity assists a vehicle traveling downhill, partially offsetting the deceleration a hump or table would otherwise produce, so the same device geometry achieves less actual speed reduction on a downhill approach than on level ground. This calculator reduces the modeled effectiveness as downhill grade increases to reflect that -- real engineering practice often calls for a taller device or closer spacing on steeper grades to compensate.

Between two traffic-calming devices, a driver has room to accelerate back up before reaching the next one -- so on a long corridor, spacing the devices closer together limits how much speed a driver can regain, sustaining a lower average speed along the whole street. Spacing them farther apart tolerates a higher design speed at each device but allows more speed variation (and potentially higher peak speeds) in between.