Effect of Bulb-outs at Intersections on Speed Reduction and Visibility Conditions in Tempo–30 Zone

Abstract

Tempo–30 zones are used with increasing frequency as a traffic calming measure. They are intended primarily to make the drivers reduce speed to an expected value and increase the safety of traffic. In the Tempo–30 zones the drivers must pay particular attention to vulnerable road users (VRU) who can travel across the street at any place. The drivers are warned of the traffic calmed area ahead of them by the specific street and junction geometry and the street furniture, compelling them to reduce the driving speed. One of the most often used design measures used for junctions located in Tempo–30 zones is narrowing of the junction legs and extending their corners. Reducing the driving speed increases the driver’s central vision area resulting in quicker and easier spotting of pedestrians about to cross the roadway. Bulb-outs (a.k.a. build-outs) provided at junction corners along the intersection legs shorten the pedestrian crossing distance and offer a better view onto the oncoming vehicles. They are usually combined with on-street parking lanes dedicated to local residents. Taking the above facts into account, the author carried out speed measurements on a chosen downtown street located in a Tempo–30 zone. In the survey several speed measuring devices were deployed to automatically measure and record the operating speed and volume of traffic at different test locations at individual intersection legs at the same time. At least one hundred (100) speed measurements were taken per measurement site and direction of traffic. During the survey the weather was dry and such was the pavement surface. The results were subjected to statistical analyses. The statistical inference part of research included, inter alia, nonparametric test of independence and median test performed to confirm the null hypothesis H0 that the measurement results do not depend on the measurement site location. The alternative hypothesis H1 – was confirmed in almost all cases, meaning that the speed distribution parameters actually depend on the measurement site location. The locations of the measurement sites were chosen considering the direction of traffic, bulb-out location and the approach and departure conditions. The measurement data and the statistical test results have confirmed that in Tempo–30 zones bulb-out corners have an actual effect on speed reduction on the approach to the junction, enhancing the driver’s ability to spot any pedestrians about to cross the street. The speed data analyses showed that raising the pavement by 7 cm along the traffic calmed street in Tempo–30 zone had only a minor traffic calming effect. With the bulb-outs in place, the vehicles parked before the junction do not obstruct the pedestrian’s central vision area, thus enhancing their ability to spot any oncoming vehicles.