Saw the tour in '09 in the Alps. The road generally closes to vehicles an hour or two before the first listed time in the program (I think this is the first commissar's pass). L'equipe do a race paper (free) and souvenir packs (not free) then after a bit race caravan comes through an hour or so ahead of the first riders; that's the sponsors vehicles. Ranges from flat-back trucks with people throwing stuff off them to modified vehicles - the year we saw it was Skoda Yeti launch so had cars with 15 foot high Yetis on the roof. That's the time you get the booty - I got a KOM hat, some Skoda stuff, various bits of preserved food.
TT might be different but the peleton was through pretty fast even on a climb, but the excitement builds with more cars and the helicopters coming through, then the bunches of riders. All over pretty fast then the broom wagon, a police car and road is open again.
Top tips - prepare to be outside for many hours, prepare to take ages to get out. Cycle or walk for a bit if you can, the gendarmes will wave you through unless something is actually about to happen on the road, and enjoy it - it's an amazing experience! A smartphone will help you keep tabs on what is actually going on as well.
Oh, and get out the way of the vehicles behind the riders if it's not barriered off. They are very close behind the riders, the guy next to me got clipped by one of the cars as he was too busy taking pics and not fast enough to jump back.