Un début de réponse pour le chiffre :
Extrait d'un carnet de route
http://www.internenettes.fr/decouv/66.html
" Le lendemain, avant de quitter Kingman, nous visitons le musée de la Route 66. Il est très différent de celui de Clinton. Il est essentiellement axé sur l'histoire de la route 66. Nous apprenons pourquoi elle s'appelle la « 66 ». Il s'agit d'une grille qui sert à numéroter les routes. Cette route, qui suit le 35e parallèle, correspond aux numéros 62, 64, 66. C'est le 66 qui a été retenu. "
Sur Wikipedia :
Championed by Tulsa, Oklahoma businessman Cyrus Avery when the first talks about a national highway system began, US 66 was first signed in 1927 as one of the original U.S. Highways, although it was not completely paved until 1938. Avery was adamant that the highway have a round number and had proposed number 60 to identify it. A controversy erupted over the number 60, largely from delegates from Kentucky which wanted a Virginia Beach–Los Angeles highway to be US 60 and US 62 between Chicago and Springfield, Missouri. Arguments and counter-arguments continued and the final conclusion was to have US 60 run between Virginia Beach, Virginia, and Springfield, Missouri, and the Chicago–L.A. route be US 62. Avery settled on "66" (which was unassigned) because he thought the double-digit number would be easy to remember as well as pleasant to say and hear.
Pour Route, c'est aussi un mot anglais qui sous entend une notion de voyage, de périple, peut-être est ce la raison ?
A voir l'article de Géo :
http://www.route66trip.com/actu/GeoRoute66-No228-1998.pdfPatrick