For anyone who is having trouble finding traffic to complete the challenge there is a simple fix!
Go into pause menu then across to horizon life and select Horizon solo. That’s it! Way more traffic thus the challenge can be ticked off the list.
Hope this helps (it drove me nuts)
I did it on the highway while getting the Retro Hatch Ultimate Speed skills, though I had to slip between traffic that was following each other closely since the AI oddly seems to only use one lane.
That was the problem. But if you go into solo then go on the highway there is way more traffic and using most of the lanes. It was driving me crazy!