Traffic Signals

When are traffic signals installed?

Traffic signals are used to control traffic at busy intersections and at pedestrian crossings.

Signals are expensive items and must therefore be used selectively and strategically.

They provide significant benefits to road users, but cannot solve all our traffic problems. In some instances they may attract or divert traffic into residential areas, lead to an increase in rear-end collisions and may create unnecessary delays for road users. But they can be an excellent remedy if well designed and used at appropriate locations.

Before installing traffic control signals an intersection must meet established criteria.

The review includes the

• volume of side street and opposing major road traffic
• volume of cycle and pedestrian traffic
• access for side street vehicles and pedestrians
• safety record of the intersection
• proximity of other traffic signals
• function of the major road in the arterial road network.

The installation of traffic signals will not necessarily reduce the number of crashes, but will alter the type and severity of those that do occur

• right angle accidents are generally reduced by about 70%
• pedestrian accidents are also reduced.
• rear end accidents may increase, especially on high volume left turn movements.

Investing in traffic signals

The provision of traffic signals involves a high capital and ongoing cost.

The main components of a traffic signal installation are

• signal lanterns
• signal poles
• signal controller
• vehicle sensor equipment embedded in the road
• pedestrian push buttons
• appropriate signage
• line marking (lanes, direction arrows, stop line)

More information is available in our brochure A network of signals and sensors (JPG)

Capital costs of signal equipment are

• $80 000 - $120 000 for intersections
• $40 000 - $80 000 for pedestrian actuated crossings.

Roadworks are often also required to increase the intersection’s capacity and can add significantly to the capital cost.

Routine maintenance and electricity costs may be more than

• $5 000 a year for intersection signals
• $3,000 for pedestrian actuated crossings.

Connection to the ACTS (Adelaide Coordinated Traffic Signal) System adds cost for telephone line rentals to transmit the data between the controller boxes, the regional computer and the central computer located in the Traffic Control Centre.

Ongoing operating costs also include costs for

• trimming trees
• replacing blown globes
• repairing the frequent accident damage to signal equipment (the local controller box can cost up to $20 000)

Traffic patterns continually change and all signals need regular monitoring through site visits and interrogating the ACTS system to check their effectiveness.

Traffic signals in South Australia

Transport SA maintains

• 419 intersections,
• 250 pedestrian crossings
• 74 koala (school) crossings.

580 of these sites are connected to the Adelaide Coordinated Traffic Signal (ACTS) system, and the remaining 90 sites are isolated locations which are stand-alone or are remotely monitored eg Mount Gambier and Pt Lincoln.

There are approximately 38 000 lamps used in traffic signals. The ACTS system fault mechanism can report blown or damaged globes to the Traffic Control Centre (TCC) for an immediate response.

In remote areas such as Port Lincoln, signals are monitored by a computer in the Traffic Control Centre which dials up the location daily and provides similar features to the ACTS system. Operating times can be changed remotely and lamp failures can also be reported.

Transport SA replaces up to 30 globes a day with lamps replaced twice a year as part of our routine maintenance program. All faults requiring immediate site attendance are generally attended to in less than an hour and with a maximum delay of two hours.

Most traffic signal lanterns use 240v lamps. These are gradually being replaced by 10v lamps, which use

• up to 60% less power
• last twice as long
• provide greater safety to maintenance personnel.

The fluorescent and neon lights in the advisory signals (eg the flashing white “No Right Turn” signs) are also being replaced with fibre optic signs to

• conserve energy
• provide greater illumination
• give a better definition between the on and off states.

Transport SA is trialing new lanterns using Light Emitting Diodes (LED), which have the advantage of

• expected longer life (up to 10 years)
• reduction of 85% of the present power use.

The improved performance of these lanterns will minimise the occurrence of failed lantern displays and result in

• reduced energy and maintenance costs
• reduction of greenhouse gas emissions
• improved level of safety to road users.

Alternatives to traffic signals

Before installing any traffic signals, we consider all possibilities including roundabouts, stop signs and give way signs.

Roundabouts

A well-designed roundabout is a proven safe and efficient form of intersection control.

Roundabouts

• reduce the relative speeds of conflicting vehicles
• involve simple and clear give way requirements.

Roundabouts may be appropriate at intersections where traffic volumes on the intersecting road are such that

• stop or give way signs or the T-junction rule would result in unacceptable delays for the minor road traffic
• traffic signals would result in greater delays than a roundabout
• intersecting roads have roughly similar traffic flows and a high proportion of right turning traffic
• intersection has more than 4 roads.

Other alternatives are pursued when

• a satisfactory geometric design cannot be provided due to insufficient space, unfavourable topography or unacceptably high construction costs
• traffic flows are unbalanced with high volumes on one or more approaches and some vehicles would experience long delays
• a major road intersects a minor road and a roundabout would result in unacceptable delays to the major road traffic
• there is considerable pedestrian activity and it would be difficult for pedestrians to cross either road due to high volumes.

Stop and give way signs

Stop and Give Way signs are used to control traffic at intersections by allocating priority to traffic on one of the intersecting roads.

Give Way signs are provided

• at all intersections with 4 or more legs
• at any 3-way intersection where the layout is not clear regarding the T-junction rule ie at a Y junction. In all other cases, Give Way signs are not required if the T-junction rule operates satisfactorily.

Stop signs are provided

• in place of Give Way signs on any controlled approach where the visibility of traffic on the major road is severely restricted.

A history of accidents may prompt requests for Stop signs. However, where visibility is adequate

• accidents can be reduced more effectively by eother meansg improved linemarking
• use of Stop signs can lead to driver disobedience, which will lead to a lack of credibility in Stop signs.