percentages of passenger-car trips of various lengths in a normal highway movement, and concerning the percentages of all car owners having annual incomes of various orders, was also available as previously set forth in this report. And finally, the Bureau was also in possession of information concerning the volume of motortruck and bus traffic on various main highways and in various sections, which, though not so complete as the available passenger-car traffic data, was still sufficient for the purpose.
As a first step in estimating their probable traffic, the selected highways were assumed to be free highways of limited access, but with access points located as they probably would be in a toll system. Guided by this assumption, an estimate was made, for each section of the proposed routes, of the amount of traffic the new facility would probably attract from existing free highways located at various distances from it and approximately parallel to it.
The first consequence of the assumed condition was to exclude as potential traffic for the limited-access route that part of the movement on any parallel free highway composed of trips shorter than the assumed distance between access points. Since vehicles making such shorter trips would be forced by the limitation of access to travel further than necessary between their origins and destinations they would not use the new facility even if it were immediately adjacent to the existing free route.
In estimating the percentage of the known volumes of traffic on parallel free routes that, for this reason, would be excluded from the limited access routes, the facts concerning the distribution of trip lengths developed by the highway planning surveys of 11 States, provided helpful guidance. These facts are presented in plate 5 and corresponding vehicle-mile data are generalized in the single curve on plate 10. Referring to this curve it will be seen that if the interval
