In general there are several main conditions which warrant the installation of safety barriers along roads:
(i) It protects vehicles from hitting a roadside object (e.g. bridge pier, sign post, walls etc). Conversely, it protects the roadside object from damage by collision of vehicles.
(ii) It avoids the crossing over of vehicles over central median.
(iii) It protects the vehicle from falling down a steep slope (more than 3m high)
(iv) A poor record of accidents involving run-off vehicles.
This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.