One of the main justifications [citation needed] for transport improvements is the amount of time that travelers will save.
Using a set of values of time, the economic benefits of a transport project can be quantified in order to compare them to the costs (thus forming the basis of cost-benefit analysis).
In particular, savings (or, for that matter, increases) in travel time form part of the change in consumer surplus for a transport project.
In practice, time spent traveling on certain modes (especially train, but also bus and car passengers) can sometimes be used to carry out some work, while time spent using certain other modes (especially car drivers, cyclists, and walkers) cannot be used to carry out work.
Generally, it is not clear what the elasticity should be,[6] and from a theoretical standpoint there is no reason why the income elasticity for private travel should be unity, since it is a matter of personal preference how individuals or households allocate additional income to purchasing time savings.