So.. how are the cars in the caravan going to keep in contact with each other?

Comunication:
It has been agreed that we should have some form of comunication between the members of the roadtrip, the methods of comunication that have been decided upon are:

FRS Radio.
The most popular of these radios is the Moterola TalkAboutTM

FRS was created specifically for the use of families and small groups. This service allows the group to use a small, easy to use, and relatively inexpensive two-way radio for the purposes of voice communication between members of the group. These radios, in many cases, give the group a choice of channels to use, to help avoid interference from other groups, as well as other features. You do not need a license, or pay any type of "user fees" or subscriptions. You simply purchase a radio, and batteries, and then you may immediately use the radio.

An FRS radio can cost anywhere between about $40 to $130, depending on features and capabilities. Accessories and other components vary in price. These are Per-Radio Costs

A much cheaper option is to purchase a radio, or a set of radios off of e-bay or one of the other auction systems online. Moterola FRS radios are selling on e-bay for 50% - 75% of retail.
Check This link for an e-bay listing of FRS Radios

FRS faq

Side-By-Side comparison of diferent FRS radios (not compleatly up to date)

For those of you that wish to use radios other than standard FRS radios (eg. Hand Held UHF Ham Radios) here is a list of the frequencys that FRS uses:
Channel Frequency Channel Frequency
1 462.5625 8 467.5625
2 462.5875 9 467.5875
3 462.6125 10 467.6125
4 462.6375 11 467.6375
5 462.6625 12 467.6625
6 462.6875 13 467.6875
7 462.7125 14 467.7125