Traffic Collisions and/or driving offences can be reported via a call to 101 or via the Dorset Police website – https://www.dorset.police.uk/ro/report/rti/rti-beta-2.1/report-a-road-traffic-incident/
(Clearly use 999 for reporting emergencies, serious road traffic incidents etc)

Traffic-related near misses can be reported via the Dorset Council website – https://www.dorsetcouncil.gov.uk/roads-highways-maintenance/road-safety