Overpasses and underpasses: effectiveness of crossing structures for migratory ungulates

Published source details Simpson N.O., Stewart K.M., Schroeder C., Cox M., Huebner K. & Wasley T. (2016) Overpasses and underpasses: effectiveness of crossing structures for migratory ungulates. Journal of Wildlife Management, 80, 1370-1378.
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This study is summarised as evidence for the following.

	Action	Category
	Install overpasses over roads/railways Action Link
	Install barrier fencing and underpasses along roads Action Link

Install overpasses over roads/railways

A replicated study in 2010–2014 of five crossing structures at two sites along a highway in Nevada, USA (Simpson et al. 2016) found that more migratory mule deer Odocoileus hemionus used overpasses than underpasses to cross a road. More mule deer crossed the road across two overpasses (234–4,007 deer crossings/overpass/season) than through three underpasses (44–629 deer crossings/underpass/season). Crossing structures, 1.5–2.0 km apart, were located at important crossings for migratory deer. One site had one overpass and two underpasses. The other had one of each structure. Overpasses, made of concrete arches, were 31–49 m wide and 8–20 m long. Cylindrical underpasses were 8 m wide, 28 m long and 6 m tall. All structures had soil bases. Fencing, 2.4 m high, deterred deer from accessing the highway between crossings and extended 0.8–1.6 km beyond crossings at each site. Crossings were monitored, during six to eight mule deer migratory periods (between autumn 2010 and spring 2014) using camera traps, over 10 weeks in each migration (15 September to 1 December and 1 March to 15 May). Cameras were positioned 12 m apart along crossing structures.

(Summarised by: Ricardo Rocha)
Install barrier fencing and underpasses along roads

A study in 2010–2014 of two sites along a highway in Nevada, USA (Simpson et al. 2016) found that underpasses, in areas with roadside fencing, were used by migratory mule deer Odocoileus hemionus to cross a road, but less so than were overpasses. Fewer mule deer crossed the road through three underpasses (44–629 deer crossings/underpass/season) than across two overpasses (234–4,007 deer crossings/overpass/season). Crossing structures, 1.5–2.0 km apart, at important crossings for migratory deer, were completed by August 2010 (August 2011 for one overpass). One site had two underpasses and one overpass. The other had one of each structure. Underpasses, 8 m wide, 28 m long and 6 m tall, were oval in cross-section. Concrete arch overpasses, were 31–49 m wide and 8–20 m long. All structures had soil bases. Fencing, 2.4 m high, deterred deer access to the highway between crossings and extended 0.8–1.6 km beyond crossings at each site. Crossings were monitored during eight mule deer migratory periods (autumn 2010 to spring 2014), using camera traps, over 10 weeks in each migration (15 September to 1 December and 1 March to 15 May). Cameras were positioned 12 m apart along crossing structures.

(Summarised by: Ricardo Rocha )