Two solutions for minimising traffic light damage

The research project Traffic lights: Collision avoidance strategies/damage mitigation with iMOVE participants the Department of Transport and Main Roads (Queensland) and Queensland University of Technology (QUT) has recently been completed. The final report for the project is available for download below.

This study identified practical engineering solutions that could significantly reduce the time and cost of repairing damaged traffic signal posts. When traffic signal posts are struck in crashes, intersections often require extended work zones that disrupt traffic flow, increase delays, and raise costs for road users. The study evaluated two alternative wiring and pole design approaches aimed at simplifying repairs and shortening work zone duration.

Alternatives

After a literature review and discussions with TMR, two alternatives were proposed and approved for investigation and cost benefit analysis.

1. BAU

The first alternative proposed was the business-as-usual (BAU) case. This proposal includes the use of existing traffic signal infrastructure but with several changes including: including relocation of the daisy chaining din-rail connector and use of retention socket poles for vulnerable single pole signals.

2.Networked signal control

The second alternative involved more significant variation with networked signal control. This proposal included a traffic signal control cabinet with network control master and simplified intersection wiring for extra low voltage power and data, in addition to sealed waterproof enclosures with network slaves in adjacent pits at signal poles, or at signal poles with short low-count spur cables and retention socket poles for single pole signals.

Benefit cost analysis

The California Life Cycle Benefit/Cost Analysis Model was employed here as it is widely used and easy to understand. The methodology provided estimates using relationships from the Highway Capacity Manual.

The researchers used the following four categories for analysis with the report providing a table of results for each:

1. Vehicle Operating Cost – which included all factors associated with fuel and non- fuel (e.g.) insurance, tires, etc.
2. Travel time cost –to estimate the number of people (on average) who occupy a vehicle and their average income in order to deem an hourly wage value for occupants of common vehicles in urban areas.
3. Crash cost – the average costs associated with fatal, serious and non-life threatening or minor injuries.
4. Emissions cost – the gaseous chemicals which can have negative effects on human health and the environment.

The first proposed alternative, BAU, confirmed that as the number of upgraded intersections increased so too did the benefits. The researchers posit that the benefit-cost ratio for upgrading 20 traffic signal posts with the proposed first and second alternatives are 4.8 and 1.35 respectively with a 7% discount rate.

The work also showed the benefits of the first and second alternative were identical because both provide the same impact in terms of traffic flow effects including travel time and its reliability, vehicle operating cost and vehicle emissions.

Strong economic case

A life-cycle benefit–cost analysis for upgrading 20 intersections showed:

• First alternative:
  • Benefit–Cost Ratio: 4.81
  • Net Present Value: $2.67 million
• Second alternative:
  • Benefit–Cost Ratio: 1.35
  • Net Present Value: $0.87 million

Most benefits come from reduced travel time, improved travel time reliability, and lower vehicle operating costs.

Crash data analysis

The current analysis focuses on the impact of different road speed limits on traffic crashes with traffic posts. In order to establish likely contributing crash factors, researchers analysed data provided by Queensland Department of Main Roads and Transport (TRM) on crashes that damaged traffic posts in the period from 2007 to 2022.

The TRM data showed that there were 8, 848 traffic intersections in Queensland with 8, 438 not experiencing any crashes involving a traffic signal. From the data, researchers were successfully able to use coordinates to locate corresponding intersections in 521 of the 528 crashes relevant to this work.

Amongst the findings were:

• Most crashes (525 of the 528) happened in South-East Queensland
• Crashes were widespread within this region
• Speed limits can and do influence intersection crashes. As anticipated, the number of crashes involving traffic posts increased with the average speed limit. The authors note however, that standard crash and traffic data is necessary to complete a statistical analysis.

Feedback from other jurisdictions

Researchers for this project contacted 10 representatives of various Australian and NZ state government departments responsible for traffic signalling, seeking feedback. Two substantive responses were received.

Firsl, feedback from ITS Solutions Development within the Victorian Department of Transport and Planning advised that Victorian intersections operated a 51-core ring circuit which allowed the pole cables to be isolated in the event of damage. It reported it had previously sought potential alternatives but had not identified any other feasible solutions and had discounted the use of underground terminations. It did, however, plan to look at retention socket poles in the future.

The second response was from Roads ACT Traffic Signals Group. It noted that aside from some legacy sites the ACT used 51 core cables. It advised that it was not aware of traffic signal controllers based on intelligent networked signal lanterns as an area of heavy investment.

Recommendations

The work here shows that the first alternative is a great investment and ensures shorter work zones during future repairs of traffic signal posts damaged by road crashes. Most savings will involve travel time and vehicle operating cost savings,
with an investment in this alternative also benefiting transport and non-transport users. The second alternative would introduce further advantages, such as simpler installation and cabling requirements.

For these reasons, the researchers encourage TMR pursue the development of the technology needed to implement both of the proposed alternatives to improve traffic pole posts.

Download the final report

Download your copy of the final report, Collision avoidance strategies and controlled failure process for traffic signal posts, by clicking the button below.