Why did the turtle cross the road? Because an underground tunnel made it safe to do so.

In 2016, the Wisconsin Departments of Transportation and Natural Resources partnered with the University of Wisconsin-Stevens Point to construct a tunnel beneath Highway 66, hoping to cut down on high turtle mortality rates, reports Robert Mentzer for Wisconsin Public Radio.

The tunnel, with Jordan Pond on one side and wetlands on the other, was a noble venture, but the turtles had no way of knowing it was a crossing point rather than a dark and potentially dangerous hole. So Pete Zani, herpetologist and associate professor of biology at the University of Wisconsin-Stevens Point, installed aluminum flashing outside of each opening, which would reflect the sky and let turtles know that there’s a light at the end of the tunnel. Zani also installed grates above the tunnel to make it less shadowy, and a small cul-de-sac in a nearby piece of the fencing to encourage turtles who had missed the tunnel to turn around.

Zani and his team found that in the first year after construction, 85 percent fewer turtles were killed on the road, and no baby turtles were among the casualties. In the last few years combined, only 40 turtles died, compared to 66 deaths in 2015 alone.

That’s great news for local turtles, of course, and it’s great news for local humans, too. The intersection in question is always busy with truckers, commuters, and families en route to Jordan Pond, and turtle crossing can exacerbate traffic congestion and increase the chance of accidents.

Not all turtles have caught on, however, and it looks like some might never get the memo. Zani found that about 30 percent of snapping turtles and 20 percent of painted turtles make it through the tunnel, and those numbers have been consistent each year since construction. “They either get it or they don’t,” Zani told Wisconsin Public Radio.

Other animals are getting it, too. As part of the experiment, Zani set up a turtle-wrangling program in which students monitored trail cameras for turtle activity outside the underpass. In photos captured by the cameras, they noticed that rodents, mink, skunks, raccoons, and even house cats were traveling by turtle tunnel.

[h/t Wisconsin Public Radio]