Drivers have been warned to be on their guard after a series of thefts across West Yorkshire.
Police said suspects have been following cars before bumping into them at low speed, and stealing handbags and cash while they are distracted.
Thieves have been targeting elderly women with the crime over the last few weeks in Leeds, Kirklees and Bradford.
Detective Inspector Neil Thompson, of Leeds District Crime Team, said: "This is a relatively unusual method for criminals to use to commit robberies and theft. While we don't want to cause unnecessary alarm we do want to make people aware of what has been happening so they can be suitably on their guard.
"We would advise people to be vigilant and exercise caution if they are bumped while out in their car, and to keep their valuables safe and lock their car when they get out to speak to the other driver."
Two men, aged 33 and 38 from Leeds, were charged this week with robbery and theft in relation to two incidents.
Det Insp Thompson added: "We are continuing to carry out enquiries into these offences and have made arrests and charged two people but we will be continue to do everything we can to identify anyone involved in using this kind of tactic.
"If anyone has seen anything suspicious in relation to this type of incident or has any information that could assist in identifying those involved, we would ask them to contact us immediately."
Anyone with information is asked to contact Leeds District Crime Team at Elland Road via 101 or call the independent charity Crimestoppers anonymously on 0800 555 111.