The interesting part to me is that the dead-straight second leg of the Rambouillet Voiturette circuit was the Dourdan record-stretch which hosted such prominent sprint and record meetings in period.
Concerning the piece of road used for the "kilomètre à Dourdan":
On Saturday, 3 May 1902, Willie Vanderbilt, David Bishop and Henri de Rothschild tried to break the kilometre record, at the wheel of three 40-hp Mercedes, three 6.5-litre Forties.
On Saturday morning, Vanderbilt, Bishop (Rothschild joined in the afternoon only) and the official ACF time keepers Tampier and Gaudichard had the intention to use a stretch of the road from Chartres to Bonneval, just south of Chartres near Thivars, between the kilometre stones 101 and 102 (now N10). Despite heavy rain, Vanderbilt achieved 33 4/5 seconds over the flying kilometre, and Bishop 36 flat.
Then, the party took lunch at Chartres where they were joined by Rothschild, and decided to use another piece of road in the afternoon, between Ablis and Saint-Arnoult (now D988), between the stones 38 and 37. Tampier took place by the side of Bishop, Gaudichard by the side of Rothschild, and Édouard Nobilé (correspondent for the newspaper L'Auto) by the side of Vanderbilt. Following several trials, Vanderbilt achieved 32 2/5 (111 km/h), Bishop 36, and Rothschild 36 4/5. Later in the afternoon, the weight of Vanderbilt's Forty was checked by the ACF in Paris, 930 kg. (Serpollet in his steamer had just achieved 29 4/5 on the Promenade des Anglais at Nice.)
This piece of road between Ablis and Saint-Arnoult was part of the course used for the Coupe des Voiturettes.
On Tuesday morning, 5 August 1902, Vanderbilt was back at Ablis, but at the wheel of his 9.2-litre Mors and on the western side of Ablis, on the stretch between Ablis and Chartres (now D910). Vanderbilt achieved 31, 30 4/5 and 30 4/5 in three trials. Then, he completed a flying mile in 48 2/5 (nearly 120 km/h). On Tuesday afternoon, Vanderbilt achieved 29 2/5 (122.48 km/h) over the kilometre, a new absolute record, beating Serpollet.
Among the spectators was the son of the mayor of Saint-Arnoult, and he suggested a better stretch of road, a four-kilometre stretch between Saint-Arnoult and Dourdan (now D836). Tampier and Gaudichard immediately had a look, and the road between Saint-Arnoult and Dourdan became the piece of road used for the "kilomètre à Dourdan" (kilometre stones, etc.. described for example in La France Automobile/1902/page 744). This was the stretch used by Fournier and Augières in the fall of 1902.
This stretch between Saint-Arnoult and Dourdan was not part of the Coupe des Voiturettes course.