This is a bit of a tough question, as it depends on several factors. I would in fact say that there is no possible general answer to this question.

When moving at typical vehicular speeds, the shutter speed is selected to stop motion, in this case, the photographer's motion. When you're moving at these speeds, objects closer to you appear to be moving faster than further away objects. This phenomenon is known as parallax. In addition, it would depend on whether the objects are moving to your side (like the landscape), or are more or less moving towards you (like cars in front of you).

In addition, the higher your camera's resolution, the more noticeable shake and/or motion blur will be when viewing the image at full resolution. However, for a print of a given size, shake and/or motion blur should be the same regardless of sensor resolution.

It is possible to trigonometrically calculate the apparent motion of an object at a given distance and thereby select the shutter speed based on the acceptable amount of motion blur for a given photograph, but given that you're probably going to be well past the object by the time you finish calculating, there isn't a lot of point.

So in conclusion, for close or medium distance objects, I'd try to take photos with high (1/500 seconds or less) shutter speeds and keep going higher or lower till you find an acceptable setting. For far away objects, you can go pretty low, possibly all the way down to 1/50 seconds. Eventually, you'll learn to judge the shutter speed fairly accurately depending on your sense of motion; but it will require practice and deliberate experimentation to develop that judgement.