There  have been many tragic accidents over the past few years involving vehicles sliding off the highways in and around Lillooet BC.  The 'ditches' in this part of the world are  very deep and a re- assessment of highway safety and improvements is needed to prevent further unnecessary traffic fatalities. 

The BC MInistry of Transportation website provides information on the rationale for location of CRB: 

Concrete roadside barrier (CRB) is installed along the shoulder of highways where it is justified and where funding permits. Not all locations require CRB. Some of the criteria that is assessed when determining if CRB should be installed is: shoulder width, the slope of the ditch and volume of traffic on the highway. Generally – the steeper the shoulder, the narrower the shoulder, and the higher the traffic volumes – the more likely barrier will be justified. 

from http://www.th.gov.bc.ca/popular-topics/faq.htm#contract