The Puck Test

by Guest blogger Mike Allende

Hockey is a winter sport and WSDOT is getting in on the puck action. Well, kind of. While it doesn’t involve goalies, slap shots or penalty boxes, this action may end up helping traffic move more efficiently.

Soon we’ll be giving Sensys Puck road sensors their first Seattle-area tryout when we install them on northbound I-5 near Northgate. It’s a test project pitting the youthful technology of these wireless sensors against the veteran savvy of existing loop detectors embedded in the roadway in a heated matchup to see which performs best in gathering traffic and vehicle information.

Loop detectors only work with good pavement, and funding for pavement preservation is looking grim. We still need to have reliable traffic information, so as an alternative, the plucky pucks may be the answer.
The pucks are compact and solid, just 3 inches tall and 3 inches in diameter, but don’t let their size fool you. There’s plenty of power and versatility packed in those little bodies and they hold up well under pressure, whether it’s a semi-truck or heavy rain.

What can these little guys do? Plenty. They count every car as it goes by, measuring speed, lane occupancy, gaps between vehicles, direction of travel and even vehicle length. This information is valuable in evaluating traffic flow and signal control, and letting drivers know if a road is congested or not on our flow map. That’s some MVP ability.

Inductive loops have similar skills but require pavement that is in good shape in order to work well. Pucks are also easier to handle than the gangly and awkward loops. That means they can get in position quicker, usually just 10 to 15 minutes per puck, leading to a much shorter disruption of traffic. Two loops, on the other hand, take about two hours to install.

To prepare for the puck’s debut, crews bore a circular hole in the pavement, about 4 inches wide and 4 inches deep. We want to put the puck in the best position to be successful, so crews level the holes so that each sensor gets an accurate reading.

After the pucks are in position, they're well protected with a strong epoxy. These guys are valuable and they need to be taken care of. They look sharp and impressive in their black uniforms, but at the same time aren't distracting. They'll fit right in.

The pucks aren’t going to be on their own, though. They team with a local controller, which receives transmissions and crunches data from each individual sensor. The controller can give an overall report about the presence of vehicles, average number of vehicles at any time, average speeds and lane occupancy, among other information. That’s some great teamwork! Loops also work in tandem but have to be hard-wired to a controller device, meaning there are only particular areas they can work without incurring a huge cost.

The pucks often make their appearance overnight to little fanfare, and can even be installed using a brief traffic slowdown rather than lane closures, unlike the loop sensors. They are run by battery and expect to have a long 10-year career. They’re also healthy, and unless the battery dies or there’s an epoxy failure, there shouldn’t be any other maintenance necessary. If maintenance is needed, the pucks can easily be popped out of the pavement, minimizing concrete damage. The health of loops is more questionable, as maintenance is a big issue with these guys. Some loops hold up well over time, others have to retire after 2 or 3 years. Maintenance involves digging up the concrete, dealing with the loop, and replacing the concrete, a much more invasive process. These are huge factors as pavement preservation is vital with increasing traffic. The less the ground has to be dug up, the longer it will last.

So who’s it going to be, the grizzled veteran or the intriguing rookie? The Northgate project costs about $20,000 while a comparable setup with the loops would be about $15,000. The signing bonus is higher for the pucks but add in the costs of maintenance down the road and the potential traffic disruptions and the up-front cost may be worth it. Clearly there are pluses and minuses with both, so the result of this tryout is going to be interesting and exciting to watch!