If you are looking for a map of free wifi locations around the world, I recommend you try this map at WeFi. You can search all across your route.
Also, since you mentioned you are going to Miami. If you take the Florida Turnpike, all of the Welcome Centers on the Turnpike have Free Wifi.
The Florida Department of Transportation is currently offering Free Wireless Internet (WIFI) access at all of its Welcome Centers as well as the Columbia County Rest Area facility, and the Turkey Lake Service Plaza on the Turnpike. Once the public is connected, the user will have 15 minutes of free internet access followed by a subsequent period of up to 90 minutes of paid internet access. Following that, they will have a two hour shut out period that does not permit the same laptop to access the internet again at that rest area for two hours. The WIFI is currently being provided in a test environment and will be monitored for potential permanent status for the future.
A host of other places in the area also have free wifi if you plan to stop. Here is a short list:
Target, Lowes, Home Depot, Apple Stores, McDonald's, Panera Bread, Fazoli's, and Starbucks.