Do speed bumps really decrease traffic speed? An Italian experience

Italy introduced the extensive use of speed bumps only in 1990. in an attem pt to limit the high number of fatalities involving pedestrians in urban st reets caused by the high speed of vehicles. In many countries., such device s have been the subject of careful investigations (in order to assess their effectiveness and disadvantages for the traffic circulation) and this has resulted in a number of modifications in the design to improve their perfor mance. On the contrary, no systematic and scientific studies have been carr ied out on Italian installations: moreover, the type of undulation adopted is known to produce a series of problems for some categories of users and i s not so effective in reducing speed as larger devices such as 'speed humps ' or 'speed cushions'. This paper proposes a study of the effectiveness of 23 speed bumps installed in the city of Cagliari, to this aim, a speed anal ysis was performed at speed bump locations, at the crosswalks protected by the devices and at sections of the streets where bumps are installed but fa r from them. The results show that in one third of the cases the 85th perce ntile of speed measured at the speed bumps is higher than the posted speed limit (50 km/h) and an equal percentage of vehicles travel at a speed in th e range of 45-50 km/h. No statistically significant differences were found from the comparison of speed values observed in free, bump or crosswalk sec tions of the same streets, while speed profiles calculated at four sites. w here a high percentage of braking vehicles was observed, showed a common tr end from which it clearly emerges that the effect of the device on driver's behaviour is restricted to a short spatial range (about 20-30 in before an d after the bump). The current situation thus suggests the use of more effe ctive devices such as humps or cushions, or the integration of speed bumps with other traffic calming techniques. (C) 2001 Elsevier Science Ltd. All r ights reserved.