Nighttime driver detection distances for Tasmanian fauna: informing speed limits to reduce roadkill

Abstract

Context. Roadkill is a wildlife management issue in areas where high traffic volume passes through relatively intact natural habitat. High density of roadkill in Tasmania is observed in concentrated ‘hotspots’, and local mitigation of vehicle speed may thus be an appropriate management response. Although warning signage is often advocated, this may not effectively reduce roadkill if the suggested speed does not provide sufficient time for the animal to be detected and for the vehicle to be stopped.

Aims. The detection distance at night for common roadkill species and corresponding driving speeds to avoid collision were determined. The importance of animal size and fur colour in determining detection distance was also evaluated.

Methods. Mounts of nine nocturnal Tasmanian mammal species were used to determine nighttime driver detection distances based on individual driver trials. These were converted to appropriate stopping speeds by accounting for reaction time and braking distance. Photographs and digital image analysis were used to evaluate fur brightness.

Key results. A total of 339 individual detection distances for the 9 species were recorded for 18 drivers. Detection distance differed between the species tested at both high- and low-beam headlight settings. The endangered Tasmanian devil (Sarcophilus harrisii) had the shortest mean detection distance when headlights were on high beam (60.8 m), and the second shortest on low beam (33.9 m), which corresponded to a driving speed which would permit a safe stop of 54 km h^–1 and 38 km h^–1, respectively. The greatest detection distance was for the introduced hare (Lepus europaeus): 116 m (83 km h^–1) and 50.4 m (48 km h^–1), respectively.

Conclusions. Nighttime driving speeds slower than 80 km h^–1 may be effective in reducing roadkill in wildlife hotspots. Detection distance was significantly related to fur brightness, as determined by image analysis, and not animal size.

Implications. The variation in detection distance allows species-specific nighttime driving speeds to be considered by individual drivers and by road and wildlife managers planning mitigation efforts for vulnerable species.