growth in the corridor, Jorgenson examined traffic counts in the corridor for several years before the opening of the new facilities and found that they were closely correlated with gasoline sales in Connecticut. Consequently, he used information on the growth in gasoline sales after the opening of the parkways to estimate the normal growth in traffic. Using this approach, Jorgenson concluded that the parkways generated 20 to 25 percent more traffic in the corridor than would have been expected from the normal rate of growth.

Lynch (1955) analyzed traffic in the Maine Turnpike/U.S. Route 1 corridor to estimate the traffic effects of opening the turnpike. He used information on the growth of traffic on major roads in Maine outside the corridor to estimate normal growth in the corridor. He concluded that 5 years after the opening of the turnpike, traffic in the corridor was 30 percent greater than would have been expected as a result of normal growth.

Both the Connecticut parkways and the Maine Turnpike serve high volumes of intercity trips, and both provided significant reductions in both peak and off-peak travel times.

Frye (1964a, 1964b) examined the traffic effects of the construction of the Dan Ryan and Eisenhower expressways in Chicago on the basis of a review of traffic counts and origin-destination survey data collected before and after the opening of the expressways. He found that traffic through a 5-mi screenline centered on the Dan Ryan Expressway increased by 11 percent after the expressway opened, but concluded that almost all of the increase was a result of route diversion. He found a 21 percent increase in traffic for the Eisenhower Expressway, versus a 14 percent increase in three control areas. Frye identified four factors contributing to the exceptional growth rate in the Eisenhower Expressway corridor:

  • Natural growth: the increase in traffic in the study area that would have occurred regardless of whether the new expressways were constructed.

  • Adverse traffic: an increase in vehicle miles of travel (VMT) on local and arterial streets that is necessary to get to or from expressway on- or off-ramps.

  • Diverted traffic: traffic diverted from routes outside the study area to the new expressway or to local streets and arterials for which travel conditions have improved.

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