Isivimbeli sokuphahlazeka, esibuye saziwe ngokuthi i-Guardrail, isithiyo esiklanyelwe ukuvimbela izimoto ukuthi zingaduki endleleni nokunciphisa ubukhali bokushayisana nesithiyo noma isakhiwo esiseduze nomgwaqo. Ngokuvamile eyenziwe ngensimbi, ukhonkolo, noma ezinye izinto, izithiyo zokuphahlazeka zifakwa emaphethelweni emigwaqo emikhulu, amabhuloho, nezinye izingqalasizinda zezokuthutha ezinesivinini esikhulu ukuze kuhlinzekwe ngendawo yebhafa phakathi kwezimoto kanye nezingozi ezingaba khona.