Just get a wideangle lipstick lens, a long svhs cable and some gaffers tape. I shot a 4 min chase scene in december and mounted this bad boy on the bottom of the front fork of a stunt bike. Awesome looking footage. I hooked the lens into a cheap sony digital 8 stuffed into a waist pouch of the driver and it would've been impossible to get that footage any other way.
Sticking a big camera like a pd150 or XL1 on a moving vehicle is asking for trouble if you don't have a super secure mount.
Now before anyone chimes in about quality differences, most chases are just cut cut cuts. You can get away with murder in the editing bay (but not in the shooting.
Camera angles are more important than the quality differences, which in this case were minimized due to careful post processing and colour correction.
In test screenings, I've had more than one person gawk at that footage, and it was like 5 mins to setup.
If you were really bent on it, you can get angles that no other camera can get. My advice to really sell speed is to get that camera as low to the ground as possible. I've shot stuff out of a sun roof, and it doesn't sell speed, even when going past the posted sign limits. Stick that lipstick lens under the front bumper for real cool photography.