Page:America's Highways 1776–1976.djvu/286

 came to be known as “desire lines,” showing the desired paths of all trips, either by private vehicle or transit in the area. The heaviest bands then gave indication of the desirable location of arterial routes (whether or not feasible) and the traffic volumes the trips would produce. As a check on the accuracy of the survey, the number of trips crossing a natural screen line, such as a river, as shown by the desire lines could be compared with actual vehicle counts. Generally a high accuracy was found, often over 90 percent agreement, but where the sample produced too few trips (seldom did the interviews produce too many) the screen line counts provided a basis for adjustment.

This map shows the major “desire lines” that resulted from a transportation study made in Baltimore in 1938.

It is important to recall what was recognized then—that the process was a survey. It did not produce a plan nor even the future volumes for design purposes. The only way to estimate future travel was to assume that if the planners expected the population to increase by say 50 percent, then 50 percent should be added to all current origins and destinations. It was known that that wasn’t right, for certainly population or industry would grow more in some zones than in others, but it was better than not increasing them at all, which some planners were then urging. It became apparent early in the game that a relationship existed between the use of the land and the travel it produced or attracted — and that it could be measured. But means to use this relationship in developing plans for the future were rudimentary at best. The earliest effort to apply this approach was probably in San Juan, Puerto Rico, where the travel to and from the existing airport was transferred to the expected location of a new airport in developing a freeway plan.

It remained for the development of the computer to permit exploitation of the land use-transportation relationship, given its first major test in Detroit. This new computer technology marked the turning from surveying to planning. But the application of the new technology could not have occurred without the resources of the highway department’s 1½ percent HPE funds being made available. And it could not have been implemented without conversion of the land use inventories and projections, then usually shown on maps by zones of different colors, from a qualitative to a quantitative basis. It was necessary to know the trip producing characteristics of each type of land use. It was important to know not that 280