Signal Spotlight: Improving the Resiliency of Traffic Signal Infrastructure
Among the other trials 2021 has brought to public works departments, it has been a particularly active year for hurricanes and tropical storms. Connecticut has had a number of damaging storm events over the last 10 years, with storms Elsa, Isaias, Dorian, Jose, Sandy and Irene causing significant damage to public infrastructure and private property. Widespread power outages impacted businesses and delayed the state’s return to normal operations. Many Connecticut municipalities are taking steps toward greater infrastructure resiliency, including continuity of traffic signal operations during power outages.
Dark intersections pose an immediate safety concern for the traveling public. In a New York State Department of Transportation study by Rensselaer Polytechnic Institute in 2009, researchers found that 77% of all dark signal-related collisions reported were right angle collisions, and there were twice as many collisions involving injuries at those locations than collisions with no injuries reported. Dark intersections can also cause delay for emergency responders at locations that typically provide emergency pre-emption of the signal operations and may contribute to congestion in areas with significant traffic volumes. Having a plan in place for power outages and other emergencies is essential to providing good basic service.
Stop signs are one option for providing control at a dark intersection. This requires the municipality to have a stock of temporary stop signs on hand and manpower to install the signs at the outage location(s) and collect them when the signal is back to normal operation. The MUTCD requires that the signal must flash all red upon startup where temporary stop signs are installed to avoid confusion, and the temporary stop signs must be removed before returning the signal to stop-and-go operation.
Providing backup power is another option in a power outage. There are several considerations when determining if and when to provide backup power at signalized intersections:
Portable or permanent generators are one option for providing power during short-term outages. For gas-powered portable generators, a lockable access port may be installed on the outside of the cabinet. These generators are transported to the intersection during the outage and plugged into the cabinet. Permanent generator installations typically run on compressed natural gas (CNG) and can automatically switch to generator power in the event of an outage. Florida and Utah have guidelines for operating traffic signals on generator power. Costs can range from about $1,200 to install an external generator panel to in excess of $20,000 for a CNG system.
Also known as battery backup systems, an uninterruptible power supply (UPS) with battery storage is another option for powering a traffic signal during a power outage. These systems typically cost approximately $3,000 to $5,000 plus installation.
Safety is always top priority, and it is important for personnel to wear appropriate personal protective equipment and to set up a proper work zone while installing temporary stop signs or generators at a dark intersection. While using generators, care must be taken to ensure they do not run out of fuel or pose an electrocution risk to anyone who may touch the equipment. In the case of a battery backup system, storing large batteries in a cabinet at the intersection can pose a hazard in the event the cabinet is struck by a vehicle and battery acid is released into the environment.
Maintenance considerations for generators include maintaining a supply of fuel for portable generators and regularly testing generators to ensure that they and any automatic switching equipment are operating correctly. Battery backup systems may be installed in the controller cabinet, but battery fumes and leaking acid may corrode the sensitive electronics in a controller cabinet. It is common for this equipment to be installed in a separate cabinet, typically on the same concrete pad as the controller cabinet. A remote battery monitoring system may be considered to ensure that the equipment will operate correctly in the event of a power outage.
Considerations for installing generator and/or battery backup power at signalized intersections generally include:
- Railroad pre-emption
- Emergency vehicle pre-emption
- System master controllers
- Highway ramps and single point urban interchanges
- Unique intersection geometry
- Multiple left or right turn lanes in the same direction
- History of signal malfunction due to power quality or reliability issues
- Intersections on high volume roads
- High-speed approaches
- Signal repair response time
Some states, like Maryland, Texas, Utah and Wisconsin, have state-specific guidelines for the installation of UPS devices. Other states, like Georgia, require battery backup to be installed at all locations with railroad pre-emption and make all other determinations on a case-by-case basis. The Connecticut Department of Transportation does not currently allow UPS systems at stateowned traffic signals, though municipalities may install them on town-owned signals.
More information on planning for infrastructure resilience may be found in FHWA Publication FHWA-HOP-15-024 – Transportation System Resilience to Extreme Weather and Climate Change.
If you have traffic signal systems questions, please contact:
Theresa Schwartz, P.E., P.T.O.E. – Traffic Signal Circuit Rider