The comfort and effectiveness of road humps

Areas of Expertise (1)

>>
|

Principle and objective
The speed adopted when driving over a round-top road hump will depend on the discomfort it generates (expressed in terms of vertical acceleration felt by the driver). A road hump should cause little discomfort at the design speed (V85), but sufficient vertical acceleration at higher speeds to encourage drivers to reduce their maximum speeds.

Relation between shape / speed / acceleration
Measurements have shown that a sinusoidal road hump 12 cm high and 4.8 m long induces a V85 speed of 30 km/h, as this speed on average (depending on the vehicle) generates a vertical acceleration close to the discomfort limit (= 0.5 g). Moreover, the profile of the hump plays a major part in the vertical accelerations experienced by drivers at different passing speeds: with a sinusoidal profile the discomfort increases exponentially with speed, as shown in the diagram below.

 

A
Sinusoidal access profile: vertical acceleration is small at low speeds, but increases exponentially with speed.

B
Trapezoidal access profile (steeper on-ramp): vertical acceleration is already considerable at low speeds and rises above the discomfort limit even at moderate speeds.

C
Short road hump (< 2 m): vertical accelerations are not only greater at low speeds than with a sinusoidal profile, but also decrease with higher speeds, which is exactly opposite to the desired effect.

Comparison between two road humps