Slashing speed limits doesn’t slow roadkill, study says

  This map shows stretches of highway studied to see how — and if — speed-limit reductions affected vehicle-wildlife collisions. COURTESY IMAGE
This map shows stretches of highway studied to see how — and if — speed-limit reductions affected vehicle-wildlife collisions. COURTESY IMAGE

 By Mike Koshmrl Jackson Hole Daily   January 15, 2020


Every time a multimillion-dollar wildlife overpass or underpass is  proposed to ease vehicle-wildlife collisions, one stakeholder or another  suggests enacting a speed-limit reduction instead.

It’s an  understandable, intuitive reaction. Who wouldn’t want to spend less  money on a solution that could be implemented much more easily? The  problem is, lower speed limits, especially on two-lane rural highways,  don’t work.

That’s the conclusion The Nature Conservancy and  Northern Rockies Conservation Cooperative recently reached after  assessing years worth of data from experimental 15 mph nighttime  speed-limit reductions on six stretches of highway in southwest Wyoming.

 “From the data that we looked at, we didn’t find any difference in  the number of collisions that were occurring between the reduced and  normal speed limit areas,” Nature Conservancy scientist Corinna Riginos  told the Jackson Hole Daily. “In both cases, it was because drivers were  still going quite fast. Even though they slowed down a little bit,  they’re still going pretty fast, and it makes good sense that at that  speed — around 65 mph — drivers and animals just don’t really have  enough time to avoid each other.”

Riginos, a former Jackson resident, led the research, which the Wyoming Department of Transportation funded.

Although  WYDOT slashed nighttime speed limits from 70 to 55 mph at the test  sites, motorists on average slowed down only 3 to 5 mph. The disregard  for the posted maximum speed held true across all six study sites, three  of which were selected because they bisected mule deer migration  routes, and three because they overlaid mule deer winter range.

At  the three winter range sites, 237 carcasses accumulated. There were no  fewer collisions on the test stretches — in fact, Riginos said, there  were slightly more.

Riginos similarly found no statistically  significant differences in how deer interacted on the highways during  periods of reduced speed limits. Traffic volume was the one variable  that made a difference.

“The higher the number of cars going by,”  Riginos said, “the more likely they were to have near misses and the  more attempts to get across the road the deer had to make.”

The  results were unfortunate for wildlife managers and highway departments  that have sought less costly and infrastructure-heavy means to drive  down roadkill rates. Signage, speed-limit reductions, animal-detection  systems and other collision deterrents have all been pitched as  solutions, but when it comes down to it, crossing structures such as  overpasses and underpasses — which are more than 80% effective — are the  only proven fix, Riginos said.

 “We can keep trying to layer on these things that will hopefully make  some difference to human behavior,” she said, “but there is a bit of a  leap of faith whether we’re actually going to change behavior.

“It  continues to be incredibly tempting to think that we’re going to find  an easy fix to this problem — and I would love for that to happen — but  unfortunately we just keep finding that crossing structures are the best  thing, and nothing else really comes close,” she said.

In  Wyoming, wildlife-vehicle collisions run up a societal tab of more than  $50 million annually, factoring in expenses related to human injury,  property damage and the loss of wildlife.

Riginos emphasized that  the lack of compliance torpedoed the effectiveness of lower nighttime  speed limits, but roadkill rates and speed are well correlated by recent  research: There were 61% more deer-vehicle collisions in places with  round-the-clock speed limits of 65 mph compared with 55 mph.

The  roads studied were as near as Highway 191 at Warren Bridge and as  distant as Highway 89 near Evanston. The data collection period spanned  from fall 2016 to spring 2018.

A copy of the report summarizing the research is attached to the online version of this story at