Chapter 5
The Design of Highway Intersections
5.1 Introduction,
A highway intersection is required to control conflicting and merging streams
of traffic so that delay is minimised. This is achieved through choice of geo-
metric parameters that control and regulate the vehicle paths through the
intersection. These determine priority so that all movements take place with
safety.
The three main types of junction dealt with in this chapter are:
Priority intersections, either simple T-junctions, staggered T-junctions or
crossroads
Signalised intersections
Roundabouts.All aim to provide vehicle drivers with a road layout that will minimise confu-
sion. The need for flexibility dictates the choice of most suitable junction type.
The selection process requires the economic, environmental and operational
effects of each proposed option to be evaluated.
The assessment process requires the determination of the projected traffic
flow at the location in question, termed the design reference flow. The range
within which this figure falls will indicate a junction design which is both eco-
nomically and operationally efficient rather than one where there is either gross
over or underprovision. Different combinations of turning movements should
be tested in order to check the performance characteristics of each junction
option under consideration.
The starting point for any junction design is thus the determination of the
volume of traffic incident on it together with the various turning, merging and
conflicting movements involved. The basis for the design will be the flow esti-
mate for some point in the future – the design reference flow (DRF). It is an
hourly flow rate. Anything from the highest annual hourly flow to the fiftieth
highest hourly flow can be used. For urban roads, use of the thirtieth highest
flow is usual, with the fiftieth highest used on interurban routes.
Use of these figures implies that, during the design year in question, it can be
anticipated that the design reference flow at the junction will be exceeded and a