ROADS. ROAD JUNCTIONS AND INTERSECTIONS.

1) A road junction, as the term is generally used, is the point at which one road meets another; an intersection is the point at which two or more roads cross each other. Both junctions and intersections are, of course, the worst danger spots in a road system.

2) The problems of reducing danger at these points are those of cost and space. If junctions and intersections such that all classes of traffic meet each other at the same level, there is a danger of collision, not only between cars of the same class but between those of different classes. Almost complete segregation of different classes can be achieved, and the need for users of the same class to cross traffic streams, the most dangerous process of all, can be avoided.

3) The perfect example of complete segregation of different classes of traffic and of the avoidance of crossing traffic streams is the clover-leaf junction, at which no collision can occur between vehicles if the drivers of those leaving the junction can manage to avoid those already on the road which they are approaching - which is a difficult thing

4) All forms of road junction can be classified into three groups: multi-level junctions, roundabouts and flyover-junctions.

a) Multi-level junctions. The clover-leaf, the most typical of these, has already been mentioned. There is need for multi-level intersections where three conditions are fulfilled:

1. Only a small percentage of the traffic must turn to left or right;

2. The major volume of traffic is travelling on a fast through route;

3. The volume of traffic would otherwise be sufficient to justify the provision of a roundabout.

5) b) Roundabouts. Unlike multi-level intersections, roundabouts don't enable traffic to cross without dropping speed but pedestrians and cyclists cannot be segregated unless costly over- or under-passes are constructed.

6) The success of a roundabout depends greatly upon the ease with which vehicles using it can "weave" or pass from one traffic lane to another. The greater the length of the road in which the weaving can be carried out and the smaller the angle of approach of converging streams of traffic, the more easily can weaving be performed. The angle should not be greater than 30 degrees. The greater the diameter of the island, the smaller the angle of convergence.

7) c) Flyover-junctions. These have been developed chiefly at places where there are no pedestrians (and cyclists are few, if any). These "flyovers", which enable high speeds to be maintained, are extremely expensive, costing about ten times as much as a roundabout, so it is much better to have ten roundabouts at ten dangerous junctions than a single flyover at a single junction.

8) d) A combination of roundabout and flyover bridge can be of great value.

Date: 2015-01-12; view: 1198