In order for cars to appear on roads (since TS2009) the cars must now be track objects and the config.txt file of the vehicle must have the this tag:
trackside 0.01
If you want to use 1950s-Era USA Region,<kuid:425968:1319> you'll have to clone is and edit the config.txt file to replace all of the vehicles with cars that are already configured as track objects. Here are a few:
1947 Buick Blue - Traffic,<kuid:535875:1060>
1953 Chevy - Visor, Black - Traffic,<kuid:535875:1061>
1953 Chevy, Brown - Traffic,<kuid:535875:1062>
1954 Oldsmobile Lt Green - Traffic,<kuid:535875:1064>
1956 Chevy, Gold - Traffic,<kuid:535875:1065>
1956 Chevy, Mint Green - Traffic,<kuid:535875:1063>
Bob Weber