Is dash cam evidence allowed in an investigation

While it might be allowed, in many instances it won't be used. If it is a minor accident with minimal damage (from the insurer's or police point of view) there isn't going to be an investigation by either the police or the insurance company. Also a video will only show a partial picture of what actually happened. For example, a video might very well show the impact of a collision but not show who had a red light and who had green.