al between any two access points were 10 miles, at least 20 percent of the vehicle-mileage of traffic moving on an immediately adjacent free highway would have to be counted as unavailable for the limited-access route. If the distance between access points were 20 miles the excluded portion of the free highway vehicle-mileage would be increased to 40 percent.
That part of the traffic on parallel free highways not excluded from the limited-access route by the distance between accesses, might be attracted by the superior facility of the new route. Whether it would be or not would depend upon the distance it would be necessary to travel and the character of service available over existing roads from the point of origin to the new route and from the route to the point of destination, and also upon the whole extra distance entailed by use of the new facility. While the superior design of the new route, if operated as a free facility, would doubtless be considered by potential users as outweighing some extra distance, there would obviously be no advantage in its use if to reach it at one end of a trip and continue from it at the other it were necessary to travel as far over existing cross roads as the distance via a comparable parallel road directly from origin to destination. This consideration would impose a definite limit upon the lateral distance over which the superior facility of the new route would attract traffic from existing parallel roads. Of the traffic moving over a closely parallel existing road it would definitely exclude only the traffic of comparatively short trip length; of the traffic moving over more distant parallel roads it would definitely exclude larger parts as the separating distance increased.
Beyond these definite limits use of the new facility would involve greater travel over existing roads than would be necessary for direct travel by a comparable existing road from origin to destination. Under this condition use of the new facility would be inspired only by a quickly satisfied curiosity. Within the limits mentioned, use of the new facility would not require greater travel over existing roads than would be necessary to travel entirely over such roads from origin to destination but would increase the total travel distance. Under this condition, the decision whether or not to use the new road would depend upon an individual appraisal, by potential users, of the competing attractions of short distance on the one hand and a better facility for part of a longer distance on the other. The number of potential users who, under this condition, would actually choose to use the new facility would increase with reduction in the amount of extra distance involved in such use in relation to the total length of the possible trip over the new facility. Traffic of long range moving in the general direction of the new facility would obviously be attracted to it from a greater lateral distance than any short-ranging traffic. For this reason, the greatest potential usage of the new facilities would naturally be expected to be generated by the existing-road traffic indicated on plate 7 as “foreign.”
In estimating the traffic that would probably use the proposed new facilities if they were operated as free limited-access roads, these several considerations were kept in mind in a section-by-section appraisal of the probabilities with respect to the entire mileage in
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