Assessing the value of investment in road improvement
The returns from low cost engineering measures have generally been justified in terms of the first year rate of return from comparing the value of accident savings in that year with the cost of the treatment. Accident savings are so high  that more detailed analyses are not necessary.
As the safety of the network is improved and the most cost effective measures have already been applied, more detailed cost benefit assessments are required, ideally taking into account investigation and redesign costs as well as engineering costs of the new measures. Advice is given in PIARC  on techniques to be followed.
For longer life measures, such as the construction of safety barriers, net value costs (construction and maintenance costs) and net value of the benefits from estimated accident reduction discounted over the life of the project should be compared.