Hai
if u redirect to car page ,did u show car details like size,cost for travel ect....
why u redirect car page,in booking page,if user click search car u can popup car page in popup window and user select car then close popup window and pass car details or any unique name to booking page.
try this links
http://stackoverflow.com/questions/17418618/creating-asp-net-content-in-a-new-page-pop-up-window[
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http://stackoverflow.com/questions/17582081/how-to-open-aspx-web-pages-on-a-pop-up-window
u can u use session but ,most of booking site it not redirect to other page,simply popup details page.or before going to book u need to show car details after that use go to booking page and select interested car name form dropdown box
Regards
Aravind