There are some US highways already set up for one way:
USfreeway1lane1wayTraffic,<kuid:131986:1144>
USfreeway2lane1wayNoTraffic,<kuid:131986:1043>
USfreeway2lane1wayTraffic,<kuid:131986:1143>
USfreeway3Lane1WayTraffic,<kuid:131986:1141>
USfreeway4lane1wayTraffic,<kuid:131986:1145>
USfreeway5lane1wayTrafic,<kuid:131986:1146>
USfreeway6lane1wayTraffic,<kuid:131986:1148>
I have not used these and may not be suitable so you could check out:
Road Car Spawner BNSF50,<kuid2:39134:101138:2>
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The one way direction is controlled via the config.txt file tags "isfreeway" (set at 1 means one way traffic) and "numlanes" (set to number of lanes).
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I'm not sure that a car generating asset can be invisible, I have not found out how anyway - so if somebody could enlighten me....
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If you get stuck I can send you some car spawning assets for one way single and multi lane that I have made - they have not been "published" yet and are subject to change.
Cheers