Plot near the highway - is it worth it?

I was more concerned about the noise than about the air. We had the same situation as you described, also with a noise barrier, the residential area was east of the highway. Depending on the wind direction or if the road was wet, it felt like the traffic was rolling right past your feet. It was especially "nice" during vacation times when the highway was busy day and night. I couldn't sleep with the window open, I could hear the noise even through the closed window, others might be less sensitive. The more disturbing thing was not the usual background noise that was always there, but the particularly loud days due to wind or wet road. On the ground floor, the noise was noticeably less audible than on the first floor - even though there were buildings in between.

Is the property west or east of the A9?

slightly east. Is there a difference? :)

The wind mainly comes from the west. On the east side of the highway, you therefore tend to have more noise and exhaust fumes.

I also think that traffic noise is more decisive than possible exhaust gases, etc. I grew up in the Ruhr area on a main road and have always lived in a larger city. Meanwhile, I find the constant traffic noise extremely disturbing and stressful. I currently live almost next to a motorway feeder and the tram runs right outside the front door. We are now building in a village, 2 km away from the motorway. It is quiet there and I am really looking forward to that. We also looked at plots in a new development area about 300 m next to the A5. You would always hear the traffic there. I looked at the city's noise protection report for the area. That would not have been for us. Sleeping with open windows there would be like it is now in the city.

Also think that the noise there is actually difficult. You can certainly get used to a certain level, but eventually it's over and it is really just burdensome. You should really think carefully about whether it is worth it.

If the terrain is rather flat, and you do not see the [Autobahn], then the noise is also not as loud. For us, the [Autobahn] is on the opposite side of the valley; houses below us hear much less of it than we do. Noise spreads funnel-shaped upwards ...