Wildlife

Reducing the Speed Limit Won’t Make Roads Safer for Wildlife

February 20, 2020

deer on road with colored lights at night
A deer rests on the side of the road along highway 49 in Nevada. Photo © James Folder/TNC Photo Contest 2018

Dusk. The road winds through this cold desert, snow piles tucked in the lee of every sagebrush like the white bottoms of the pronghorn that call this place home. Around a bend, there are fields, dry stubble dotted with mule deer. A hundred, at least, in this one spot.

With sudden purpose, a doe hops the fence and enters the right-of-way. Several others lurk behind, waiting to see what their leader will do. Her antenna-like ears are perked, scooping the hum of the car. She hesitates.

Maybe she will; maybe she won’t. I slow to give her the pause to make her decision. Her safety depends on it. So does mine.

And herein lies the premise: if drivers are moving slowly enough, they will have time to avoid hitting large animals, such as deer and pronghorn, on the road. Drivers who are driving more slowly have a shorter stopping distance – the time it takes to see an animal, hit the brake, and stop.

This premise underlies a question I have heard countless times: why don’t we just reduce the speed limit to solve the problem of animals getting hit by cars?

This is a problem that impacts some 6,000 animals every year here in Wyoming, and 1-2 million more large animals nationwide. The simplicity and logic are naturally appealing. If we can just get people to drive more slowly, we can solve this problem cheaply and easily.

But can we accomplish either? My short answer, based on years of research, is “not likely.”

elk running infront of a car
Elk attempting to cross the road. Photo © Mark Gocke

Reducing the Speed Limit Reduces Speed – But Not By Much

I recently wrapped up a large experiment where we tested the effectiveness of reducing the speed limit as a means of reducing wildlife-vehicle collisions.

We found that drivers did slow down, but only by a few miles per hour. We found no fewer wildlife-vehicle collisions or “near misses” (narrow escapes for deer and driver). Instead, we found that the number of cars on the road, rather than the speed limit, mattered quite a bit to deer.

Surprisingly, there has never been a study quite like this before. My team worked with the Wyoming Department of Transportation (WYDOT) to reduce the speed limit from dusk to dawn – when most wildlife-vehicle collisions occur – from 70 mph to 55 mph on six stretches of highway in Wyoming.

From my prior work, we knew that these were “hotspots” of high collisions rates where highways bisect mule deer winter range or migration corridors.

deer and a large highway
Mule deer along high fence, Interstate-80, Wyoming. Photo © Gregory Nickerson, Wyoming Migration Initiative/University of Wyoming / Flickr

Over two long, cold, Wyoming winters, my team braved the windswept sides of the roads to collect our data. We used radar recorders – handy little black boxes mounted on posts – to record the speed of every vehicle that went by.

We worked with WYDOT and Highway Patrol to get records of every collision that occurred and every dead deer that was found. And we used infrared thermal scopes jerry-rigged to record video footage of deer trying to cross the roads.

The essential results were quite simple: drivers very clearly slowed down when the speed limit was reduced, but only by about 3 to 5 mph – nowhere near the 15 mph reduction in speed limit.

The number of deer hit wasn’t affected by the lowered speed limit, which makes sense; if drivers hardly slowed, why would we expect any fewer collisions or dead deer?

deer running in front of car
A mule deer doe dashes across a road near Fremont Lake, Wyoming. Photo © Joe Riis & Wyoming Migration Initiative / Flickr

A Break in Traffic

These stark numbers answered our main scientific questions. But we found the real stories in the video clips – stories that played out in the dark, largely unseen by the passing vehicles.

Here we saw deer hopping fences like popcorn kernels. Deer waiting, waiting, waiting for the stream of traffic to break, sticking a hoof in the road, hesitating, turning back, circling around to try again. A doe hit, spinning like a top on the hard road surface, her companion coming to sniff the lifeless friend and wavering there, seemingly uncertain where to go next.

These scenes bring the deer’s plight to life, but they also yield important information. From the footage, we found that deer had to make more attempts to get across the road the more vehicles were present. If the deer had to make four or more attempts to cross, she had a 50 percent chance of giving up all together.

We also found that deer need at least 30 seconds between consecutive vehicles to cross the road safely. Fewer than that, and they almost always wound up getting hit or in a dangerous situation.

More than 60 seconds between vehicles, and the deer almost always crossed safely. This is extremely valuable information because it tells us that, for most of every 24-hour cycle, on most roads in Wyoming and anywhere more populous than Wyoming (okay, so pretty much anywhere), the traffic volume is too high for deer to cross the road safely.

That means that getting people to drive more slowly – even if you could – may never be enough.

The reality is that roads are a major impediment to animal movements. More than the animals that get hit, I worry about the animals that don’t even try to cross. They are the hidden casualties of an increasingly fragmented world that is cutting them off from the food and habitat they need to thrive.

A moose crossing the road.
A moose crossing the road. Photo © David Gordon

How Does the Deer Cross the Road?

All of this may sound very pessimistic, but I don’t think it is. The good news is that we have a very effective solution to make roads safe for wildlife to cross – and safe for drivers too. Overpasses and underpasses built for wildlife allow animals to cross the road without risk – bringing collision numbers down 80 to 90 percent and enabling animals to move relatively freely.

These “crossing structures” are costly, but they work, and the cost of doing nothing racks up millions of dollars per year in vehicle damage, human injuries, and lost hunting revenues. Knowing that the inexpensive alternatives – as appealing as they are – don’t work, helps to galvanize people around the need for the solutions that do.

road overpass with pronghorn on top
Pronghorn cross the road on a wildlife overpass. Photo © Mark Gocke

This year in Wyoming, various efforts have raised more than $25 million towards crossing structures. Unlike many conservation challenges, this is one we can engineer our way out of. We can help conserve some of the world’s longest large mammal migrations. We can connect habitats better, making flora and fauna more resilient to climate change and other threats.

I still drive slowly at night through high animal-use areas, and I advise others to do the same. Realistically, we may never get crossing structures in all the places that really need them, just as we may never get all drivers to slow down. But I hope that through research like this, we can make these decisions based on data and not just wishful thinking.

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20 comments

  1. The author states that drivers do not slow down much in a reduced speed area of the highway. That clearly indicates that drivers in Wyoming (and of course most likely in other states) violate the law by not slowing down> However the author does not seem to be much concerned about this fact.

    1. I don’t think it is that he is unconcerned as just stating a fact. I remember when the speed limit (at least in my state was reduced from 70 to 55 for several years. It didn’t work as most still went at least 70 and often over 90. Yes it was breaking the law but so many people did it that the police on the highways couldn’t do all that much about it. The speed limit went back up to 70.

    2. Heinz – thank you for your comment. Unfortunately, it is pretty well established that most drivers drive above the speed limit; one study showed that only 23% of drivers on rural two lane highways were driving at or below the speed limit. So our finding that drivers did not comply with the legal speed limit was not terribly surprising. – Corinna

  2. Right! Only the “crossing structures” like wildlife overpasses/underpasses & protected migration routes can help to lower the collision rate.

  3. I live and drive in an urban area, Washington DC. Rock Creek Park which virtually dissects the city supports white tail deer very well. Driving on Oregon Ave on the perimeter of the parking, I’ve seen families of deer in the part, waiting to cross the road at night. I stopped to let the family pass, they saw me and refused to cross, until I in my car had moved on. Granted the speed on that road was 25-30 mph, easy to see deer, easy to stop. I guessed the deer had figured out cars were dangerous …. and wouldn’t cross the road until I was GONE. Interesting study going in Wyoming.

  4. Here in the midAtlantic region, the deer population is (as you probably know) a real traffic problem (for them and the traffic). I, personally, have been jumped into (while sitting still at a stop sign) twice and have hit deer three times over the past 40+ years of driving the region. Multi-layered problem … and unlikely to be resolved by crossings, sadly.

    🙂 Linda

  5. I am from Southern California. It is good to read this article. There needs to be more of these articles out there so people know to slow down and help is needed to raise money for crossing structures either over or under. Wyoming, I am sure there is a big need multiple times during the day and night. Here in Southern California, not as much but still needed.
    Thanks very much for this article,

  6. Make underground roads& railways below forests and allow freedom for animals to move.

  7. One question…is there is difference in wildlife use if the “crossing” structure is an underpass (culvert) or overpass? And, which is more expensive to build

    1. Overpasses are more expensive. There are differences in use, and these depend largely on the species of animal involved. Pronghorn prefer overpasses, whereas mule deer prefer underpasses. Elk and moose also prefer overpasses but will use underpasses. The wider and more daylight is visible in an underpass, the more likely animals are to use them!

  8. Over and under passes are the best solution if there is enough money to build them. Unfortunately they take 75 years to amortize and if the use patterns change due to changing land use, they can become a white elephant in suburban areas.

  9. Hi it’s Monica yes we do need something done about speeding. They should be more wildlife crossing signs and. Something done with fast driver that speed and tailgate. My response is respect wild life give it space.

  10. Overpasses and tunnels are the answer. We’ve known this all along. Why isn’t it built in for new projects and installed in existing? Compassion and common sense saves lives, both human and animal.

  11. Nothing is going to be a 100% perfect solution but overpasses for animals seems to be the best especially in high risk areas. My brother and I used to travel to Wyoming to hunt and fish after we had retired and would stay for a month at a time never hit an animal but both of us came close a couple of times even when driving slower (40-45 ),but when an animal comes out of the dark suddenly into the middle of the road it is hard to avoid. And the pucker factor goes up exponentially.

  12. The author is not a law officer, nor passing judgment, but pointing out facts of behavior by humans and deer.

  13. In the meantime speed cameras could enforce the reduced speeds that you tried to achieve ie a camera and a sign ‘speed camera zone next … miles’. After all the goal is to reduce speed not generate tickets.
    Halting traffic at peak times for a minute or 2 every 15 – 30 minutes would create good gaps in the traffic for animals to cross safely as well, while presumably they wouldn’t try to cross with a slug of traffic passing.
    Here in NZ, deer, pigs etc are pests and persecuted, consequently they are rarely seen near major roads. From what I’ve heard not many slow down for penguins but they sure take note of seal signs after hitting their first bull seal!

  14. The key is enforcement. I know for a fact that if I go slower I am better able to respond to wildlife crossing in front of me – and I spend many hours driving at night in forested areas. To say it doesn’t matter because people don’t slow down is just saying we are not enforcing speed limits. That is different than saying it doesn’t work. Wildlife overpasses are great, especially on identified corridor routes, but not practical to install along tens of thousands of miles of high speed roads.

  15. Thanks for sharing it with us. Hope people will be aware of the logic it makes in reducing the speed limit.

  16. Seems like there may be an opportunity to develop wildlife traffic pauses for highway traffic to come to a stop every so often, guided by computer and linked to wildlife detection devices