J, you are correct, the 1908 course ended at the then western terminus of the Motor Parkway near the Bethpage Lodge at Round Swamp Road. The drivers made a left turn and went about 200 feet on Round Swamp Road before turning on to Manetto Hill Road (now called Old Bethpage Road).
Despite the American Automobile Association's October 1906 goal of never again using public roads for the Vanderbilt Cup Races, the 1908 course was made up of 9 miles of the Long Island Motor Parkway and 14.46 miles of public roads including; Round Swamp Road, Old Bethpage Road, Manetto Hill Road, Planview Road, Jericho Turnpike, and Ellison Road.
This survey from my collection shows how the Motor Parkway ended at Round Swamp Road in June 1908.The Bethpage Lodge was located in the red section north of the parkway.
This is an August 1908 view of the Motor Parway looking west from Round Swamp Road.
You can take a lap of the 23.46 mile course for the 1908 Vanderbilt Cup Race in this 10-minute film. Most recent film update: 11/25/2009
December 1, 2009 Update: This is a pdf description of the 23.46 mile course as published in the August 8, 1908 issue of Auto Topics.
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