Asset management

Table of Contents

3.3 Traffic Signals – Unable to Detect Vehicles

Inductive loop detectors at traffic signal sites are used to trigger a phase and to optimise traffic signal control operations. Inductive loop detectors are placed in-ground near the stop line and they are subjected to stresses from vehicles. Loops can get damaged once the pavement protecting them cracks or erodes to expose the detector. Premature damage often occurs if the loops are not properly installed.

Inaccurate or broken detectors will either fail to register a phase call or provide inaccurate inputs to the traffic signal control algorithm causing sub-optimal operation. Traffic signal control protocols allow the signal controller to compensate when a detector fault is suspected and alternative phase settings are implemented.

The current practice of scheduled site investigation and on‑condition‑based maintenance was considered beneficial and needs to be continued. In general, detailed guidance cannot be given on the frequency and intensity of operational maintenance as it is dependent on the specific nature of the equipment, its environment and the level of service it is required to perform.

However, the approach to installation and commissioning of signals needs to be modified to ensure that premature failures are minimised. This also includes further training of technicians, particularly when dealing with termination points.

It was also recommended that fail-safe protocols be implemented in traffic signal controllers such that faulty or unavailable detector are compensated operationally. Further, the ability to interrogate historical detectors readings to quickly identify potential detector faults needs to be considered. Road agencies need to develop their own application of the RCM tools to suit their environment and situation.

An example of the RCM process applied to traffic signals where the signals do not detect vehicles is presented in Case Study 4.

Case Study 4: RCM Process for Signals where the Signals are Unable to Detect Vehicles
  • Inductive loop vehicle detectors
  • To accurately detect and count vehicles as inputs to traffic signal control
Functional failure
  • The sensors are unable to detect vehicles
  • The sensors are sending inaccurate readings
Most prominent failure mode
  • Pavement cracks or erodes to expose loop cables. Loop lifts up out of pavement and loop frequency changes, therefore loop cannot detect vehicles
  • Loops exposed or poorly terminated due to incorrect installation
Failure effect
  • Vehicles are undercounted or over-counted, resulting in sub-optimal signal control leading to longer delays at intersections
  • Turning vehicles not detected and turning arrow not activated, resulting in long wait and increased risk-taking by vehicles
Determine criticalityFailure consequence
  • Not severe to severe, depending on location
  • Most likely
  • Critical (yellow)
List possible task (if applicable)On-condition maintenance task
  • Conduct on-condition survey and document level of pavement wear
  • Inspect condition of surrounding pavement for water ingress, pooling water and exposed loops. Repair pavement as required
Scheduled restoration/replacement task
  • Restoration as part of pavement rehabilitation
Change operating procedure
  • Activate ‘fail safe’ mode whereby detector is left on if terminal fault is found in the loop/leads to over‑counting, but at least signals change (turning arrow)
Change in commissioning (installation to site)
  • Revise installation process to ensure loops are correctly installed, with no part of the loop protruding from the pavement
  • Inspect commissioning immediately after installation, and periodically thereafter
  • Document handover between installation contractor and service operator, including inspections
  • Document correct termination techniques and ensure proper training of technicians
Equipment redesign
  • Interrogate Sydney Coordinated Adaptive Traffic System (SCATS) (or other traffic signal management system) counts for irregularities and reoccurring zero counts to identify failed loops
Call-out maintenance
  • Conduct call-out repairs as currently practised
  • All of the above tasks should be implemented