b) “rumble devices” which involve surfacing the carriageway in materials which create noise or vibration when crossed by vehicles and thus warn drivers of approaching hazards;
c) “bar markings” which comprise lateral road markings (lines at right angles to the road) on high speed approaches to urban junctions; the lines are increasingly closely spaced as the junction is approached and create a visual effect such that drivers slow;
d) road texture and colour on the approaches to critical locations (junctions, pedestrian crossings etc); and
e) linking of traffic signal timings at successive junctions to control and maintain a desired safe speed of traffic progression.
6.7.6. On lesser roads, a wide range of physical traffic calming measures for speed control (and in some cases, to limit traffic volumes) has been used, particularly in European cities. Typical measures include:
a) pedestrian refuges which narrow the effective road width, control vehicle overtaking and do not permit vehicles to reach high speeds
b) road-speed control humps which reduce vehicle speed. These can take various forms including humps (i) with gradual vertical slopes which cause vehicles to slow, (ii) with sharp vertical slopes which require vehicles to more-or-less stop and (iii) which combine up and down stream slopes with a flat central area (used since they are slightly more “bus friendly” than conventional humps)
c) road narrowing such that some classes of vehicle cannot use the road (such as trucks)
d) road narrowing such that only one direction of vehicles can pass at one time – thus opposing vehicles must give way
e) chicanes such that vehicles have to following a tortuous route through a short section of road and thus must reduce speed
f) raised junctions comprising a plateau or flat topped road hump built across an entire junction
g) planting which can be used to change the perceived width of a road to cause traffic to slow
h) and many similar measures.
6.7.7. It is undoubtedly the case that most of the foregoing measures reduce traffic speeds on less important roads. However, it is not clear if in developing cities they might:
a) introduce new hazards – for example, in developed cities, where road narrowing or chicanes have been used, there is anecdotal evidence that the measures can be hazardous;
b) result in increased environmental damage as vehicles slow or stop (in most cases as a result of the poor design of speed humps in developing cities) at humps and accelerate immediately after, thus increasing emissions; and
c) result in increased accidents due to poor design – the “absolute stop” road hump currently used in many developing cities is, again anecdotally, reported to increase the likelihood of increased “nose-tail” vehicle collisions.
6.7.8. It should be stated that the foregoing views are largely subjective and result from discussions with city traffic practitioners but are disputed by some professionals. It is clear that while the principle of traffic calming on less important roads appears to offer advantages:
a) more research is needed to determine the impacts of such measures in developing cities; and
b) design standards for speed humps and other devices and criteria for their appropriate application in developing cities, are needed.
6.8.1. Pedestrians (particularly children) and cyclists are among the most vulnerable road user groups. Since both “modes” should be encouraged on grounds of (i) positive poverty impact, (ii) reduction in traffic congestion and (iii) positive environmental impact. A positive attitude is needed within a traffic strategy to provide facilities to improve their safety. Traffic management possesses a wide range of techniques for protection of pedestrians and cyclists from conflicts with general traffic including the following (see Chapter 7 for a full discussion on pedestrian and bicycle measures):
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