Loftus
If you want to go to the most southerly part of St Pete's beach, called "Pass-a-grille" there is a toll road which leads off of I4, its very short. The toll's probably pay for the bridge which gives a short-cut from the long way round.
It joins St Pete's at the huge pink hotel called the Don Ceasar (sp) If you drive south of the hotel then take one of the short west bound side roads, they all lead on to the seafront.
This is a very quiet local's beach. There are some good facilities there. Cafe', Loungers, Cabannas, Rest room's, showers, etc.
There is also a very famous restaurant called "The Hurricane". We have had two very good meals there. Their Snapper is truly wonderful.
And
Goofyish is right about the Southern Skyways bridge. It is quite a sight and worth the diversion to travel over it, or if like me, miss the St Pete's beach turning and have no choice!!
My, I rambled on a bit here, sorry. I hope this helps in some way.
Have a great day,
Paul