S.C.
At 18 months my son spent a lot of his home time naked (if he was naked he'd use the potty and I rather look at a baby bum than change diapers!) and if he was cold he WOULD bring M. his clothes and or a blankie. You can also start on baby signs.
Anyway, my son and I both tend to run hot so we usually dress fairly lightly BUT I keep a light jacket, his extra winter coat, and a hat and gloves in the car just in case! I also keep a blanket by his car seat where he can reach it to pull over himself if he wants-- which seems to have more to do with how tired he is than the temp.
For long outdoor activities LAYERS are the way to go-- then you can add and subtract as they move around and heat up or slow down and chill....
Personally I think keeping some stuff in the vehicle is a good idea. Then you're covereed if the weather changes, or you guess wrong, or if the car beaks down on the side of the road when it's COLD!
I also leave an extra T shirt, sweatshirt, sweatpants, hat and mittens in his daycare cubby. Then he's covered there in case of misjudgement, forgetfullness or weather change!