Route 1 enters into Florida with little fan fair and no town for a few miles. A small town emerges finally; Hillard, and it is the only town you see until you get to Jacksonville. From now on, it is going to be hard to stay on course. Route 1 follows the coastline, but A1A follows it closer. Be aware that traffic on US 1 will be slow; A1A will be slower.
Jacksonville is still inland several miles; you won't actually be close to the Atlantic coast until St. Augustine.
Once you get to Daytona, you will be next to the beaches for a long, long stretch. Don't expect traffic to move at any great speed.
If your plans include Orlando, you will want to take I-4 from Daytona into Orlando. Otherwise, continue down US 1 as it hugs the magnificent wide beaches through Titusville, Merritt Island and Melbourne.
You've traveled so far... stop and walk a few of the glorious public beaches all along the coast. Jensen, Juno and Jupiter beaches are incredible and there are rarely any people there! As you go further south, this will no longer be the case.
Ft. Lauderdale Beach is long, but not very wide, but you'll see plenty of tiny bikinis and drunken college students. Fear not; they are all friendly and there are many things to do in Ft. Lauderdale, including a great IMAX theater and Riverwalk.
Between Ft. Lauderdale and South Beach are many beaches; one blends into another. The only sign you'll see on the beach itself is "no glass bottles" and in Hallover State Park, "Be aware that you may see nude people".
To get out to South Beach, you must take I-195 or I-395 in Miami. A nice side trip is to go out to Biscayne National Park and go snorkeling, or go west to Everglades National Park and see more alligators than you ever dreamed possible. It is possible, and enjoyable to do both of these parks in one long day.
From Miami, continue south on US 1 all the way to Key Largo, the entrance to the incredible Keys!