Most studies now use SP, though large programs often contain some RP work as a check and recent studies have shown RP to give slightly, but consistently, higher estimates of time values. It has been customary to state all time values as proportions of either personal or household incomes. Recent studies showed values increasing with household income, but not proportionately. This exhibits an arc elasticity of 0.47, which is similar to the average elasticities found in the UK and slightly higher than that found in Sweden. Few countries have an equivalent research basis.
Business travel poses special problems.
Many business travellers do not have fixed working hours so that it is difficult to know whether the time savings will be devoted to extra work or to more leisure. Furthermore, it is not always clear who makes the travel choice decisions and who incurs any increased money cost in choosing superior modes. A formula which includes the value of working time, corrected for the possibility that some work will be done during travel and the amount of time saving which accrues as extra leisure, suggested by Hensher, has proved somewhat difficult to calibrate. It is therefore recommended that business travel time should be treated in the same way as other working time savings. Non-work trips have always been treated differently.