Each year, vehicle-animal collisions are one of the leading reasons for crashes in Colorado — a rate that increases the further west you go.

Since 2010, animal-related crashes have accounted for around 3% of all incidents in the state, as reported by the Colorado Department of Transportation. In the agency’s northwest region, they account for around 10%.

A 2019 study of these incidents by the state’s transit agency and Colorado Parks and Wildlife found that 60% of wildlife-vehicle accidents in Colorado happen on the Western Slope. 

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CDOT releases quarterly roadkill data for the state. These counts rely on reports from its road maintenance crews and roadkill app as well as an app from Parks and Wildlife.

Where the reports come from vary slightly depending on where you are in Colorado. In 2023, carcass reports in the west and northeast came predominantly from maintenance crews (between 70% to 80%). In the Front Range and southeast, nearly half were from these crews, with the other half coming mostly from the Parks and Wildlife app.

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Due to the sourcing, the data comes with a caveat that the counts are underreported — though by how much is unknown — and can have inequalities in terms of where reports are made.

While the department warns that these numbers are intended for information only, they provide a glimpse into where and which animals are being struck.  

What did roadkill reports look like this summer on the Western Slope? 

In its latest report, detailing July to September, the department’s two western regions — Regions 3, which encompasses the northwest, and 5, the southwest, — saw the largest volume of roadkill.

The southwest region had 536 reports, or 31% of the total, during these three months, while the northwest had 430, or around 25% of the total.

This aligns with data from the past two years. In 2023, Region 5 had 2,445 total reports, around 150 more than Region 3.  In 2022, the southwest had 2,470, around 300 more than the northwest.

The reports for July to September are also closely aligned with the total numbers reported in the previous quarter between April and June.

In every region across the state, mule deer account for the largest percentage of animals hit and killed. Parks and Wildlife estimates that 2% of the state’s mule deer are killed annually in vehicle collisions — making the road-kill rate greater than the annual hunter harvest. 

From July to September, mule deer accounted for 37% of the roadkill in the northwest.  This was followed by black bears, which made up 17% of the reports, unknown carcasses, which were 15% of reports, and raccoons (7%).

Of the 74 black bear roadkill reports in Region 3, over half (41) occurred on Interstate 70. Seventeen took place on Colorado Highway 82 — the 85-mile-long stretch of road that starts in Glenwood Springs and runs through Roaring Fork Valley and over Independence Pass before ending south of Leadville. 

In the previous quarter, extending between April and June, mule deer accounted for even more, 66%, of the carcasses reported in the region. There were fewer bears reported, however, making up only 3% of the total.

In the southwest, around 68% of the roadkill reports were deer for the last two quarters.

While the summer is a busy time, looking at past years’ data, the Colorado Department of Transportation typically reports the most roadkill in October and November as the weather starts to shift with the daylight hours.

Where are the high-risk areas for wildlife collisions?

While the quarterly reports provide a snippet of information on the deadliest highways for animals, the state agency notes that it should only be used to study the same stretches of road over time.

Typically, some of the highest reports in the northwest come from sections of I-70, Colorado Highway 9, Colorado Highway 82, U.S. 50 and U.S. 40.

From July to September, in the northwest region, U.S. 40 — which, in this region, runs from the Utah border near Dinosaur and traverses to the border of Grand and Clear Creek counties — accounted for 21% of the carcasses reported, I-70 for 18% and Highway 82 for 8%. Over half of the reports during these three months on U.S. 40 occurred between the Routt-Moffat county line and Milner. 

This was a higher percentage for U.S. 40 in the region compared to the previous quarter when it accounted for 13% of reports from April to June. In the late spring to early summer period, I-70 had more reports (around 23% of the total). Around 6% of the reports came from each Highway 9 and U.S. 50 during this period. In this region, Highway 9 runs from I-70 at Silverthorne north to Kremmling. U.S. 50 runs from the state border with Utah in Mesa County through Delta and Gunnison counties in this region.

Looking at larger trends over time, this data has been used to help identify high-risk highway stretches in Colorado for wildlife-vehicle collisions, including the 2019 study of the Western Slope.

This study found that the majority of high-risk sections were in the northwest region. Among those identified as the highest priority in this corner were stretches of Colorado Highway 13, both between Meeker and Craig, as well as north and south of both towns, U.S. 40 both west and east of Craig, and I-70 between Rifle and Glenwood Springs.

Parks and Wildlife also reports that I-70 in Eagle and the stretch of State Highway 82 between Glenwood Springs and Aspen are among the high-risk areas in the state.

Ultimately, the goal of the bigger report, and the quarterly roadkill data, is not only to reduce roadkill, but also to guide decision-making around wildlife mitigation.

Currently, momentum around Colorado is building for several projects to build safe passages for wildlife. Bolstered by the success of the passages built on Highway 9 south of Kremmling in 2016, projects on East Vail Pass and Highway 82 from Glenwood to Aspen are among those in the state seeking funding to build infrastructure for wildlife.