Dark intersections can take an outsized toll on both drivers and services. Last year's Hurricane Irma exemplified the costs: with more than half the traffic lights in Miami-Dade County out, road crews scrambled to deploy limited numbers of generators; police and sheriffs departments were stretched thin trying to direct traffic manually and writing tickets to drivers mishandling the outages; and accidents were caused by confused drivers blowing through dark intersections. While a hurricane like Irma doesn't happen every day, any dark intersection is an accident waiting to happen, and two-thirds of those are injury accidents, according to a New York DOT study. For the sake of drivers, DOTs, and first responders, transportation departments need strategies to minimize the occurrence and duration of traffic signal outages.
The most common cause of dark intersections is power failure: the power goes out and either the intersection has no power backup system or the batteries in that system have run out. The obvious first line of defense is to be sure each intersection has an uninterruptible power supply (UPS), but some UPS are better at keeping the lights on than others. To avoid dark intersections, here are three factors to consider
- Battery management: With most UPS deployed today, if the cells in the lead-acid battery string are unevenly charged, the stronger batteries tend to overcharge the weaker ones, shortening the life of the battery string even further. A UPS with a battery management system can compensate for uneven states of charge. The batteries in the UPS will last longer, and if a prolonged outage happens, field crews can plug in available batteries with different states of charge with no negative effect on battery life.
- Ease of battery replacement: Replacing the heavy lead-acid batteries in many UPS systems is an involved process. Maintenance staff need to remove wires, exchange the heavy batteries, and retorque all the screws down. Instead, consider UPS systems that use alternatives such as lightweight nickel-zinc battery panels that can be unplugged when depleted and replaced in seconds using a simple barrel connector, so the system can be cold-started to quickly restore backup power to the intersection.
- Battery shelf life: Generators are typically the second line of defense for prolonged traffic signal power outages, but generators are costly and involved to move and install. In a large-scale emergency, setting up generators is going to leave more intersections dark longer. Unfortunately, lead-acid batteries require trickle charging to maintain them on the shelf, and shorter operating life due to unbalanced strings makes them a poor source for secondary backup power. But new nickel-zinc batteries don’t have to be trickle-charged. They only lose 3% a month on the shelf, and with battery management by the UPS, uneven charging won't shorten battery life in operation. So, field crews now have the option of quickly and easily replacing depleted backup batteries with new batteries and leaving the generators at the shop.
As I write this, Hurricane Florence is barreling towards the East coast of the U.S., and state and city transportation departments in multiple states are bracing for the inevitable road closures and traffic control outages it will bring. Mother Nature holds all the cards, but we can be better prepared to respond. As we plan or update our traffic systems, let's work towards a future without dark intersections. We have the power. Let there be lights!