In most cases, during daytime and downwind if the highway, the air is back at urban background levels at around 400 m from the centerline of the road. Things get bit more complicated during nighttime, but I'd say that's the big picture. And as always with things like this, the decay is exponential, so e.g. 150 m away from the road you have like three times lower concentrations than at 100 m from the road. I'd be happy to buy a home that was more than 300 m away from the nearest highway.

Source: used to be a part of quite a few mobile studies of urban air quality and pollution gradients from highways.