So you would be against storing the data for a few days so if a non-traffic crime is detected later (a murder for example), the traffic recordings can be used to help in the investigation? I think that's useful too.
If the police reach the data before it's deleted after they know it may have evidence of a crime that's ok (realistically this is rare, but I can imagine it happening with paired cameras that measure speed over a long distance of road. You could also get lucky where a false positive image happened to capture a different crime), but we shouldn't be storing surveillance data that tracks everyone's movements on the off chance that it has evidence of a crime. The harm outweighs the use at that point.