I live in Tokyo, and the air is not that clean close to highways: large diesel trucks pollute a lot, and also small motorbikes/scooters pollute horribly because they don't seem to require any emissions controls at all.

The main thing keeping the air clean here is the proximity to the bay, along with the fact that there just aren't that many private cars in the first place, since most people take public transit and don't drive because there's nowhere to park.

Large trucks do not pollute a lot (there are strict standards to that matter). While they do pollute obviously, there is no viable substitute to it. EV truck is a dream at this point in history.

Amount of private cars in Tokyo is huge. Pollution near expressways in rural japan far from bays is next to nothing, so having it close to ocean does help a little.

Small motorbike/scooters are not allowed on expressways.