Surely you'd feel or even hear that something is off?
You'd like to think so. A few months ago I was walking alongside the A38 in Brum (40 limit) and heard a thwok, thwok, thwock, from behind. I turned expecting to see an old beaten up Fiesta. It was a 4 year old Honda Civic Type R with 18's and his front left was completely flat! To the driver it must have felt like he was driving on cobbles. Must have been pulling heavily to the left but he kept going.
I don't know about you guys but I can tell if one of my tyres is a little under pressure before the Tyre Pressure Warning flags it!