Multiple ways... I have shot from the back of trucks filming cars, people running, and sceneries. One thing you will absolutely need if filming at 24-30 frames per second is a gimbal/stabilizer. This means that you do not intend to slow down the footage in post. Not using a stabilizer will not give you a smooth shot as any vibrations from the road (even smooth drops), will cause shaking. And as you are not able to stand and hold the camera, your ability to further maintain the camera from moving is limited (Your legs provide a large part of stabilization for hand held shots). Even if you shoot with a camera with internal stabilization, you will need a gimbal/stabilizer. You will also need a camera that can autofocus well. This means, use a camera with native lenses. They work better depending in the camera that you use. I have used a Sony a7sii and it works like a dream as it uses contrast to maintain the focus and the headlight of a car provide that guide. There are glitches at times though. People have also increased their shutter speed, but I find that it makes the quality of the footage seem unnatural .
If you are able to have a dedicated person to follow focus it allows you to get clearly focused images throughout most of the shoot. It is a bit more expensive as you would have to spend on the equipment and fee for focuser. Renting is always a cheaper option, but make sure you have a day to become familiar with equipment.
If you are going to slow down speed on post, you can record at 60 fps. Don't forget that your shutter speed would be double this amount. Slowing it down will take away from shakes but not all of it so always try to keep as steady as possible. Smaller the camera the easier. If you are able to use a sand bag to rest and position the camera, this also helps.
