Nigel is spot on.
You want to drive down roads lined with trees, bushes or grass. It's here that a higher bitrate comes into it's own as with a low bit rate, compression is upped and detail is lost - the trees / bushes can become smudged (used to be macro blocking but with the latest compression advances it now appears to be smudging), and if really pushed, the whole picture can become blurred.
In my opinion, this shot from an unknown camera, taken in a highly detailed roadside scene, shows lack of bitrate in this particular situation and thus high compression effects. (That doen't mean it's a bad camera, just that this particular scene is so challenging with detail, that it's had no option to resort to high compression given it's bit rate).
I've highlighted several parts, although there are many more places the effect of compression can be seen particularly in the trees.
In the bit by the lamp post I zoomed in on, you can see all the detail has been discarded and replaced by an area of similar colour producing just a khaki smudge. In the large tree furtherest down the road to the right (highlighted with the arrow), the codec appears to have discarded all the top branches and replaced them with a grey brown smudge, also seen elsewhere in the other closer trees. The driveway to the right which you can see is a highly detailed area of stoney mud, is reduced to just a smudge of brown colour as is the road to that side. Muddy areas of the banks where the roadside joins the bushy area, are just smudges. There are many more areas if you look. All of this in my opinion is typical of bit rate starvation as is a bit of an overall lack of sharpness:
You're unlikely to see bit rate starvation from a stationary image due to the way compression works - my understanding as an amateur is it saves the full frame in what's known as key frames and then tends to reference areas of the picture that are the same in subsequent frames to save saving all the detail again. No real change as when stationary = almost the whole picture referenced until next key frame = very little data saved = little to no compression effects . In my opinion, upping the bit rate for less challenging situations certainly can help the overall sharpness of a picture when moving. However, there is a law of diminishing returns here as in low detail situations the extra bit rate may have little to no visible effect on sharpness. That said, GoPros now use 60mbs (70mbs in protune - believe 4K and h265 compression). Yi+ (admittedly in 4K) up to 135mbs in h 264. However, storage considerations for continuous recording, mean you're probably unlikely to ever see that in dashcam.
I believe GoPros used to use 25mbs (Hero 2), so that isn't necessarily a bad rate. 50mbs is certainly better though and Y265 helps achieve that without additional storage requirements. Of course y265 @ 50mbs would be better still, but again it's storage and diminishing returns in ordinary low detail situations that dictates what many manufacturers are going to set.