IMHO if you stay alert to the traffic, walking on the cliff road during the day is not all that dangerous. I always walk facing the traffic and make sure I move as far as I can off the road when vehicles approach. Granted, there are a few spots where it seems like there is little or no room to move off the road but for the most part it is not an issue. Walking with little kids would be one thing but for a 12 year old I think it would be no problem.
The cars do come speeding along that road so when crossing the road I make sure to look both ways and make sure the way is well clear before stepping out into the road.
Wearing bright coloured clothing to make yourself more visible is also a good idea.