Install signage to warn motorists about wildlife presence

How is the evidence assessed?
  • Effectiveness
  • Certainty
  • Harms

Study locations

Key messages

  • Six studies evaluated the effects on mammals of installing signage to warn motorists about wildlife presence. Four studies were in the USA one was in Australia and one was in Canada.



  • Abundance (1 study): A before-and-after study in Australia found that when wildlife signs were installed along with speed restrictions, rumble strips, reflective wildlife deterrents, wildlife escape ramps and an educational pamphlet, a small population of eastern quoll re-established in the area.
  • Survival (6 studies): Three of five studies (including four controlled and three before-and-after studies), in the USA and Canada, found that warning signs did not reduce collisions between vehicles and deer. The other two studies found that warning signs did reduce collisions between vehicles and deer. A before-and-after study in Australia found that wildlife signs along with speed restrictions, rumble strips, reflective wildlife deterrents, wildlife escape ramps and an educational pamphlet, reduced collisions between vehicles and Tasmanian devils but not eastern quolls.



  • Human behaviour change (2 studies): Two controlled studies (one also replicated, before-and-after), in the USA, found that signs warning of animals on the road reduced vehicles speeds.

About key messages

Key messages provide a descriptive index to studies we have found that test this intervention.

Studies are not directly comparable or of equal value. When making decisions based on this evidence, you should consider factors such as study size, study design, reported metrics and relevance of the study to your situation, rather than simply counting the number of studies that support a particular interpretation.

Supporting evidence from individual studies

  1. A controlled study in 1972–1973 in Colorado, USA (Pojar et al. 1975) found that lighted, animated deer crossing signs reduced vehicle speeds but did not reduce deer-vehicle collisions. There was an average of one collision for each 57 deer-crossings when the signs were both on and off. Average vehicle speeds were lower with the signs on, but the reduction was by <5 km/h. Three deer carcasses at the highway edge (46, 98 and 107 m before signs) reduced speeds but the reduction did not differ between when signs were on (10 km/h reduction) or off (13 km/h reduction). Two deer crossing signs were installed along a 1.6-km-long highway section (with 97 km/h limit), where deer-vehicle collisions were frequent. Signs were reflective yellow diamonds (1.8 × 1.8 m) with four silhouettes of deer in neon tubing lighting across the sign. Signs were turned on and off for alternate weekly periods during January–March over four weeks in 1972 and 11 weeks in 1973. Numbers of deer crossing the highway were estimated by nightly spotlight counts. Collisions were recorded each night and morning. Vehicle speeds were measured at 0.2, 1.1 and 2.4 km behind the sign between 18:00 and 22:00 h.

    Study and other actions tested
  2. A before-and-after study in 1990–1998 in Tasmania, Australia (Jones 2000) found that following installation of reflective wildlife signs, speed restrictions, rumble strips, reflective wildlife deterrents, wildlife escape ramps and publication of an educational pamphlet, an eastern quoll Dasyurus viverrinus population partially re-established and vehicle collisions with Tasmanian devils Sarcophilus laniarius, but not eastern quolls, decreased. Results were not tested for statistical significance. Following local extinction, 3–4 quolls re-colonised within six months of installation, increasing to ≥8 animals after two years. Road-kills were similar for quolls before and after implementation (1.6 vs 1.5/year), but decreased for Tasmanian devils (3.6 vs 1.5/year). Following road widening in 1991, vehicle-wildlife collisions increased and quolls became locally extinct (from 19 animals). In 1996, large, reflective signs displaying a wallaby, and the words ‘Cradle Wildlife Zone’ were installed, along with the other five interventions. Animals were surveyed using 60 cage traps for three nights in alternate months in October 1990–April 1993. Then, 10–20 traps were set for 20–100 trap nights each April, May and July in 1995–1998. Spotlight counts were made once or twice in 1991, 1995, 1996 and 1998. Road-kills were recorded in 1990–1996.

    Study and other actions tested
  3. A replicated, controlled, before-and-after study in 1996–2000 along roads in three townships in Michigan, USA (Rogers 2004) found that deer warning signs (including some of a novel design) did not reduce deer-vehicle collisions. In one township, the overall collision rate after installing standard and novel warning signs (55/year) did not differ significantly from that before installation (69/year). At the same time, there was no change in average rates in three townships without warning signs (after: 41–62/year/township; before: 36–62/year/township). There was no significant difference in average collision rates 200 feet either side of signs on seven road stretches that just had the novel sign design (after installation: 9/year/stretch; before: 11/year/stretch). Vehicle speeds were not lower with signage than without along one road stretch and were <0.5 miles/hour lower along a second stretch. Two warning sign designs were installed around one township between October and January of 1998–2000. Eighteen novel signs, (leaping deer and car on an orange background and text stating ‘High crash area’) were installed on seven road stretches with high vehicle-deer collision rates. Fifty-two standard signs (leaping deer on orange background) were installed on other sections. Collisions, monitored by State Police, were compared in the township before (1996–1997) and after installation (1998 and 2000) and in three townships without signs. Vehicle speeds were monitored for 15–24-hour periods before (1,124 vehicles) and after installation (1,221 vehicles) on two road sections.

    Study and other actions tested
  4. A replicated, controlled, before-and-after study in 1995–2002 along five highways in Utah, Nevada and Idaho, USA (Sullivan et al. 2004) found that temporary warning signs reduced vehicle speeds and collisions with mule deer Odocoileus hemionus during migrations. Fewer deer deaths occurred after signs were installed (3–12/migration) than before (7–35/migration). Concurrently, deaths did not decline on a road section without signs (after: 3–13/migration; before: 3–11/migration). Once signs were installed, the proportion of vehicles speeding (8%) was lower than before they were installed (19%). There was no concurrent decline on a road section without signs (after: 19%; before: 25%). Signs affected speeds of heavy trucks more than of passenger vehicles. Sections of five highways, crossed by mule deer seasonal migrations, were studied. Each 6.5-km-long section was divided into two with each half randomly assigned as treatment or control. Treatment sections had temporary yellow and black warning signs (2 × 1 m) with reflective flags and solar-powered flashing amber lights installed at each end and smaller signs (1 m²) each mile. Deer-vehicle collisions were monitored daily during spring and autumn migrations, before (2–4 years) and after (1–4 years) signs were installed. Night-time vehicle speeds were monitored in 2000–2001.

    Study and other actions tested
  5. A before-and-after study in 1989–2004 along 22 sections of highway in Kansas, USA (Meyer 2006) found that deer warning signs did not reduce vehicle collisions with white-tailed deer Odocoileus virginianus. The collision rate after signs were installed (0.83) did not differ from than in the 2–10 years before signs were installed (0.78; units not clear in report, but may refer to deer killed/km/year). However, the rate over just the three years after sign installation (0.71) was significantly lower than that in just the three years before installation (1.16). Numbers of collisions closely followed trends in deer populations, which increased to a peak in around 1999 and then decreased. Deer-vehicle collision data were obtained for 22 sections of highway (section lengths not stated) across seven counties for 2–10 years before and 2–5 years after deer warning signs were installed. Timing of sign installations was not known precisely but was assumed, in the report, to have been within six months of publication of Road Safety Reports, which were mostly published in 1999.

    Study and other actions tested
  6. A replicated, randomized, controlled, before-and-after study in 2005–2008 at 26 urban sites around a city in Alberta, Canada (Found & Boyce 2011) found that warning signs reduced the number of collisions between vehicles and white-tailed deer Odocoileus virginianus. At warning sign locations, there were fewer deer-vehicle collisions after sign installation (0.4 deer-vehicle collisions/location/year) than before (1.7 deer-vehicle collisions/location/year). Concurrently, at locations without warning signs, there was no significant difference in deer-vehicle collision rates after (1.0 deer-vehicle collisions/location/year) compared to before signs were installed (1.7 deer-vehicle collisions/location/year). Twenty-six road locations with high incidence of deer-vehicle collisions were selected. Pairs of reflective deer warning signs (90 × 90 cm, diamond shape) were mounted on 3-m-high posts, 1,600 m apart, facing opposite directions, at 13 locations (randomly selected) in June 2008. The other 13 locations had no signs installed. Deer carcasses (mostly white-tailed deer but possibly some mule deer Odocoileus hemionus) were monitored within an 800-m radius of each location from June to December in 2005–2007 (before sign installation) and in June–December 2008 (after sign installation).

    Study and other actions tested
Please cite as:

Littlewood, N.A., Rocha, R., Smith, R.K., Martin, P.A., Lockhart, S.L., Schoonover, R.F., Wilman, E., Bladon, A.J., Sainsbury, K.A., Pimm S. and Sutherland, W.J. (2020) Terrestrial Mammal Conservation: Global Evidence for the Effects of Interventions for terrestrial mammals excluding bats and primates. Synopses of Conservation Evidence Series. University of Cambridge, Cambridge, UK.

Where has this evidence come from?

List of journals searched by synopsis

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Terrestrial Mammal Conservation

This Action forms part of the Action Synopsis:

Terrestrial Mammal Conservation
Terrestrial Mammal Conservation

Terrestrial Mammal Conservation - Published 2020

Terrestrial Mammal Conservation

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