Helping people with computers... one answer at a time.
There are many options for connecting while traveling, but for connectivity while literally on the road there's only one practical solution.
When we go on vacation I figure if I'm in a car for 8 hours at a time, I might as well be doing something constructive! What's best way to stay connected? WiFi? Something else?
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First: if you plan to use your computer while in motion I hope that you're not the one driving. 
Second: WiFi is a great solution when stopping at rest stops that offer it, or at local coffee shops that have WiFi hotspots along the way.
Unfortunately, WiFi is not a general-purpose, connect-anywhere solution; it has a very severe limitation that rules it out for this connectivity as you drive down the road.
However, there is an alternative.
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WiFi is a short-range technology - meant to use in a home or small area, and connects to your WiFi router that is connected to your DSL/Cable/broadband. Most laptops come WiFi ready.
If all you want to do is connect at hotspots while you're parked (hotels, McDonald's, Starbucks, what have you) then WiFi works, since that's what the hotspots provide - a WiFi interface to their own broadband connection.
Driving on the highway is different. VERY different:

There's actually nothing wrong with firing up WiFi in your moving vehicle, the problem is simply that with only 300 feet of range, there's not likely to be anything to connect to. (We'll look at one exception to this at the end of the article.) At best, you might get very brief connectivity to other WiFi hotspots you pass on the road, or the WiFi of others who might happen to have a computer running in their vehicles.
The current best approach to getting connectivity in motion, or without having to rely on WiFi hotspots along the way is to get a cellular modem and a data plan from your cell phone provider.
Cellular has two specific characteristics that make it perfect for exactly what you're envisioning:

To begin with, the range of cellular signals isn't measured in feet, it's measured in miles. That means that you don't need to be nearly as close to cellular towers as you do WiFi hotspots.

Secondly, your cellular provider has established a comprehensive network of cell towers that will provide you continuous and transparent connectivity as you drive along the highway. Before you get out of range of one tower the system "hands you off" to the next tower along the way, without interrupting your connection.
All in all, exactly like your cellular phone calls.
And this is exactly what I do.
I have a USB cellular modem from my cell phone provider that I can plug in to my laptop. Anywhere there's cellular coverage, I get connectivity. And yes, my wife's been known to work for an hour or two while I drive, heading up the highway. I'm very pleased.
There are laptops that actually include cellular broadband hardware, so you wouldn't need an additional modem. I tend to shy away from them because just like cell phones themselves, they work with only one provider. If I want to change providers I simply get a new modem, not a new laptop. In addition, since I have several portable machines, my USB modem will actually work in any of them.
I alluded to a way that WiFi might be used, and it's something I do as well: use a cellular WiFi router:

This device can be considered a portable WiFi hotspot that uses your cellular modem for internet connectivity. In a nutshell, you plug your cellular modem into one side, and the router provides a WiFi hotspot out the other. That hotspot is then available to any WiFi enabled computers in range.
Put that in your car, and you've got a moving WiFi hotspot.
It's a great way to provide connectivity for more than one computer at a time, and for computers that, for whatever reason, can handle WiFi but not the cellular modem directly.
Article C3850 - August 23, 2009 « »
August 28, 2009 5:55 AM
SpacemanBill mentions the ''USB sticks'' easily available in Canada from various providers such as Rogers, Telus, Bell. These are excellent, so easy to use and work very well. I imagine that all major carriers in the US, UK, UE, Japan use the same technology. By the way, Leo's article is excellent.
December 16, 2011 6:16 PM
Dumb Question: I have a tablet that is WiFi (no 3G/4G or USB port). Is there such a thing as a portable Wifi device available?
17-Dec-2011
December 16, 2011 8:24 PM
@yoohoo
You could get a cell phone that projects a Wifi spot, and then use the tablet off of that. So, yes, there is portable Wifi devices. I know that Verizon has them, probably other carriers too. You can even run a computer off a cellphone wifi.
December 18, 2011 6:49 AM
I'm going to look into this this week.
From what I've read this seems like it is a bit expensive. I'm going to call Verizon and see if I can have my cell enabled to do this.
Thanks Everyone! AND a BIG thanks to Leo for all of his hard work! I learn so much here!! Love this site!!! :)
December 24, 2011 5:43 PM
I've tried T-Mobile, ATT, and Verizon laptop connect..they all suck...if used in motion, especially in rural areas it keeps disconnecting and reconnecting while the car is moving..and when i try to view live video (sports, etc.), or download movies, all it does is buffer..when I complained to them, they Said they couldn't guarantee a seemless connection..I wish they had something better