It is basically confirmed that sightings will not show any pokemon if you were at driving speed after the last patch. It should be an intentional change made by Niantic.
As for Hatching, I personally feel that you will lose more distances when not going straight now. But I also find that the app will track your distance even when the screen is locked, which actually surprises me. Last Saturday I tried to keep the screen locked and only unlock it every 0.5 km. The distance, somehow, was perfectly recorded. (I didn't have a Go plus)
Nearby Pokemon not showing up in the nearby list
So my gf and I have both noticed over the last week that suddenly nearby pokemon are showing up on the nearby list in what seems to be a massive delay. We live in the county so often times one of us mans the pokemon Go in the passenger seat if we drive somewhere and nearby pokemon just aren't showing up all. Is this an intentional change to prevent GOing and driving? In addition to this, even when walking doesn't seem as if we're gaining any credit towards hatching eggs are towards our buddies. I have tried deleting the app and reinstalling the app, restarting my phone, etc. the issue seems to persist. Is anyone else having similar issues, give me any advice how to fix the issue, or tell me for a fact that it's an intended change? perhaps I overlooked it in the patch notes, but regardless my buddy and eggs are still not working right. Part of me thought that the GPS tracker isn't registering our movement properly which is why pokemon aren't appearing and the eggs aren't hatching.