How to access the internet during a road trip in the USA?

You can walk into any large carrier store - AT/T or T-Mobile in your case (since you probably have a GSM phone), and ask for a prepaid SIM card. No address proof, I don't even think they check your ID. T-Mobile usually has the best deals (value for money) and their 3G/4G network is pretty fast. Of course, I am assuming you have an unlocked GSM phone.

T-Mobile's prepaid deal is ~$50 - unlimited everything (data, texts and calls). And they have cheap add-ons for international calling.

Also of note, although T-Mobile is cheap and has pretty good coverage - the 3G/4G bands used are slightly different from world standards. They started re-framing their network - so international/non-tmobile phones will work on their 3G network - but rural areas may no have this new re-framed network. EDGE will always work though.

Also, LTE (with either ATT or TMo) is probably totally out of the question - we in the US are special and use special bands that international phones don't possess. :)

I recently traveled from Belgium to San Francisco.
I'm not sure whether it's the same as over there, but we had free WiFi in our hotel and in about every café, restaurant or tourist location we visited.

Some Belgian providers have certain deals for mobile usage in the US.
At Mobistar, there's a monthly plan with lower prices for texting, calling (in & out) and mobile data usage.
Proximus is enormously expensive abroad, especially when it comes to data usage. They do have a daily plan. For 5€/day you use it, you get a certain amount of texts, minutes and Mb data usage.
Mobile Vikings is one of the cheapest, but I don't know their plans.
I have no idea about deals for Base or other provides.

Unless it's for work, I can't really imagine why you need to have constant network access.
A lot of the people I toured with, used it for Foursquare and Facebook messages. If you really want to do that, keep it for when you arrive back at your hotel.

It depends on personal choice, I guess.

Pre-paid phone service is a minority in the US, but it does exist. Most of the major carriers provide pre-paid access, and there are also some discount carriers (with limited coverage areas) such as the one I have used, SimpleMobile.

The easiest (and possibly only) way to purchase one of their SIMs without a US address will be to go to one of their retail locations. Search the website for your chosen brand, and you should be able to find a listing of retailers in whichever city you'll be visiting.

You will likely still need to provide an address when registering the SIM and purchasing airtime, but at that point you can probably use the address of your hotel, or wherever you will be staying.

Depending on how long you're going to stay in the US, consider getting a postpaid plan. I went traveling in the US for 6 months around 2 years ago, and easily got myself a postpaid plan with ATT on the first day ($30/mo for 3GB, I think, pricey).

I gave the address of my hotel at the time, and since they couldn't run a credit check on me as a foreigner, they wanted a deposit of $500, which I received back without issue 3 months after canceling the account. I didn't mind the deposit, YMMV. Connectivity was pretty good throughout, a little spotty in national parks and other less populated areas, but fine overall.

Free WiFis exist indeed, but more often than not they come with annoying restrictions and the speed is less than staggering.

As a side note, Americans are being royally screwed by their cellular services providers. In addition to the high prices, in the US they also pay when they GET a call or a text, unless they happen to be on a pricey unlimited plan. This sounds completely insane to anyone outside of the US, where only the caller/sender pays (as logic and basic decency dictate), but it's true.

This is pretty easy in the US. Just go into a best buy or something similar and purchase a SIM-Card (no proof of anything needed)

My suggestions for a card are either MetroPCS (40$ unlimited Data, Text, Talk) or h2owireless (40$ unlimited Talk, Text, Data + 1000 Intl. SMS + 5$ intl. Credit - ~2hrs talktime). Their SIM-Cards are working in almost any Mobile Phone. You just have to register this card for the first time online (use free WiFi). And you can refill it with either a card which you can purchase in-store (BestBuy) or by credit card. You don't need proof of anything at any point!

Another way would be to use the huge number of free WiFi's which are offered by a lot of Cafes, Restaurant etc. ...

I am from Europe/Austria and just 5 months back I was using a T-Mobile prepaid SIM card on my iPad mini, which I am then using as a wifi hotspot for my other devices. It was just about 50USD for some gigs and I was very happy traveling from San Francisco up until Seattle. There are of course some spots along highways for example, where coverage was very poor. But it was always enough to use Google Maps. In the cities it was even great for doing Facetime Audio or Facetime Video calls back to Europe/Austria. Enjoy your stay ;)

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.

You go to a T-Mobile store for example and get a prepaid sim card for a month for like $50. As simple as that. Coverage is a different story.

I don't have any personal experience with their service, but AIO Wireless looks like the best option and would be the first choice I'd try if switching from a contract to prepaid service. Unlike most other prepaid providers where you get a small data allowance then get cut off, AIO gives 3G speeds until you hit your usage cap, then just drops you down to 2G speeds (which are still usable for anything but video/voip, basically) with no usage limit. They're also using AT&T's network (and, I believe, owned by AT&T now) which actually has coverage everywhere, unlike T-Mobile's which is flaky outside of major urban areas.

There are also prepaid carriers using Verizon and Sprint's networks, with very good and very poor coverage, respectively, but since they're CDMA rather than GSM you'll have to use the device they provide rather than BYOD and chances are slim that it'll support tethering out of the box.

Just simply freedompop...go to their website and you can have personal 4G Internet access hub in your pocket wherever you go.....yes, I mean wherever you go....Just go to their website and find out for yourself. www.freedompop.com...Basic plan is FREE 500MB of Data.

You don't need to go into Cafe filled with teens and kids who wants to text and surf Facebook at the cafe... Or drive for miles and miles just to get access to the Internet. So, you can read your email... :)

I am in no way affiliated with this company. I am just one of their customer.

I recently had a trip in the US where I needed a temporary SIM card. At T-Mobile I had no problem getting a pre-paid SIM card with 4G internet and they didn't even ask for my address.