Another significant factor of the physical environment was South West Africa's limited road network. The main arteries for SADF bases on the border were two highways leading west to Ruacana and north to Oshikango, and a third which stretched from Grootfontein through Kavangoland to Rundu. Much of this vital road infrastructure was vulnerable to guerrilla sabotage: innumerable road culverts and bridges were blown up and rebuilt multiple times over the course of the war. After their destruction PLAN saboteurs sowed the surrounding area with land mines to catch the South African engineers sent to repair them. One of the most routine tasks for local sector troops was a morning patrol along their assigned stretch of highway to check for mines or overnight sabotage. Despite their efforts it was nearly impossible to guard or patrol the almost limitless number of vulnerable points on the road network, and losses from mines mounted steadily; for instance in 1977 the SADF suffered 16 deaths due to mined roads. Aside from road sabotage, the SADF was also forced to contend with regular ambushes of both military and civilian traffic throughout Ovamboland. Movement between towns was by escorted convoy, and the roads in the north were closed to civilian traffic between six in the evening and half past seven in the morning. White civilians and administrators from Oshakati, Ondangwa, and Rundu began routinely carrying arms, and never ventured far from their fortified neighbourhoods.