I you want static vehicles, just add them to your route in Surveyor.
If you want vehicles for traffic, edit the config.txt file for the region of your route. Your will find a list like the one below that lists the kuids of vehicles that you want in traffic. You can change or add to this list:
car0 <kuid:447264:1132>
car1 <kuid:447264:1133>
car2 <kuid:447264:1134>
car3 <kuid:447264:1135>
car4 <kuid:447264:1136>
car5 <kuid:447264:1137>
car6 <kuid:447264:1138>
car7 <kuid:447264:1139>
car8 <kuid:447264:1140>
car9 <kuid:447264:1141>
car10 <kuid:447264:1142>
car11 <kuid:447264:1143>
car12 <kuid:447264:1144>
car13 <kuid:447264:1145>
car14 <kuid:447264:1146>
car15 <kuid:447264:1147>
car16 <kuid:447264:1148>
car17 <kuid:447264:1149>
car18 <kuid:447264:1153>
car19 <kuid:447264:1154>
car20 <kuid:447264:1155>
car21 <kuid:447264:1160>
car22 <kuid:447264:1163>
car23 <kuid:447264:1164>
car24 <kuid:447264:1165>
car25 <kuid:447264:1166>