As with priority junctions and roundabouts, the basis of design in this instance
is the ratio of demand to capacity for each flow path. In addition, however, the
setting of the traffic signal in question is also a relevant parameter. The capac-
ity of a given flow path is expressed as its saturation flow, defined as the
maximum traffic flow capable of crossing the stop line assuming 100% green
time. Traffic signals will normally operate on the basis of a fixed-time sequence,
though vehicle actuated signals can also be installed. The fixed-time sequence
may be programmed to vary depending on the time of day. Typically, a sepa-
rate programme may operate for the morning and evening peaks, daytime off-
peak and late night/early morning conditions.
5.5.2 Phasing at a signalised intersection
Phasing allows conflicting traffic streams to be separated. A phase is charac-
terised as a sequence of conditions applied to one or more traffic streams.
During one given cycle, all traffic within a phase will receive identical and simul-
taneous signal indications.
Take,for example, a crossroads intersection, where north-south traffic will
conflict with that travelling from east to west. Since the number of conflicts is
two, this is the number of required phases. With other more complex intersec-
tions, more than two phases will be required. A typical example of this is a cross-
roads with a high proportion of right-turning traffic on one of the entry roads,
with movement necessitating a dedicated phase. A three-phase system is there-
fore designed. Because, as will be shown further below, there are time delays
associated with each phase within a traffic cycle, efficiency and safety dictate
that the number be kept to a minimum.
The control of traffic movement at a signalised junction is often described in
terms of the sequential steps in which the control at the intersection is varied.
This is called stage control, with a stage usually commencing from the start
of an amber period and ending at the start of the following stage. Stages
within a traffic cycle are arranged in a predetermined sequence. Figures 5.28
and 5.29 illustrate, respectively, typical two-phase and three-phase signalised
intersections.
5.5.3 Saturation flow
The capacity for each approach to an intersection can be estimated by summing
the saturation flows of the individual lanes within each of the pathways. The
main parameters relevant to the estimation of capacity for each approach are:
Number, width and location of lanes
The overall capacity of an approach road is equal to the sum of the satura-
tion flows for all lanes within it. It is assumed that the average lane width
The Design of Highway Intersections 133
