Main central island
The main central island will have a radius of 2 m, the maximum allowable for
an ICD of 28 m, again to allow the safe movement through the junction of large
articulated vehicles. In order to limit the circulatory carriageway, therefore, a
low profile subsidiary central island should be provided, extending the radius of
the central island out to a total of 5 m. The subsidiary island provides deflec-
tion for standard vehicles while allowing overrun by the rear wheels of articu-
lated vehicles.5.5 Basics of traffic signal control: optimisation and delays,
5.5.1 Introduction
Tr affic signals work on the basis of allocating separate time periods to conflict-
ing traffic movements at a highway intersection so that the available carriage-
way space is utilised as efficiently and safely as possible. Priority can be varied
with time through the cycle of the signals. Within urban areas in particular, in
situations where a road has a number of intersections along its entire length,
signal linking can be used as a method for allowing almost continuous pro-
gression of traffic through the route.
In the UK the decision to install traffic lights at a given junction is arrived at
through assessing in economic terms the reduction in delay resulting from it
together with the forecasted improvement in accident characteristics and placing
these against the computed capital and operating costs. In the UK the usual
sequence for traffic signals is red, red/amber, green and amber. In Ireland, the
sequence is red, green and amber.
The installation of traffic signals is justified by the need to: Reduce delay to motorists and pedestrians moving through the junction
Reduce accidents at the junction
Improve the control of traffic flow into and through the junction in particu-
lar and the area in general, thereby minimising journey times
Impose certain chosen traffic management policies.Tr affic signals can however have certain disadvantages: They must undergo frequent maintenance along with frequent monitoring
to ensure their maximum effectiveness
There can be inefficiencies during off-peak times leading to increases in delay
and disruption during these periods
Increases in rear-end collisions can result
Signal breakdown due to mechanical/electrical failure can cause serious
interruption in traffic flow.132 Highway Engineering