9.1 Introduction

Traffic signals are used to regulate and control conflicts between opposing vehicular or pedestrian traffic movements. Without the use of signals at some sites the major flow would dominate the junction, making entries from the minor road impossible or very dangerous. At other sites the minor road might interfere with the flow of major road traffic to such an extent that excessive congestion would occur. Traffic signals cannot only improve junction capacity, but can also improve road safety.

Modern traffic signal controllers utilising microprocessors are reliable and flexible and can be programmed to handle multiple phases and other features, including:

• pedestrian facilities;

• pre-determined fixed time;

• vehicle actuation including 'hurry calls' actuated by excessive queues on certain arms, public transport or emergency vehicles;

• links by cables or cableless linking facilities to other nearby signalled junctions or pedestrian crossings and integration into an urban traffic control (UTC) system, including adaptive systems such as split cycle time and offset optimisation technique (SCOOT).

9.2 Fixed-Time Control

In fixed-timed control the proportion of green time assigned to opposing arms is pre-set in accordance with 'historic' traffic data. At most sites, traffic flows vary throughout the day. Typically inbound flows to a city are high in the morning peak period and outbound flows are high in the evening. Different green splits may, therefore, be required at different times of the day or year.