The Independent Office for Police Conduct (IOPC) is investigating four separate fatal collisions involving police patrol cars.
All occurred in the last week in police pursuits or emergencies.
Paul Radcliffe, an 18-year-old boy, was killed in the first of the crashes on 19 January, when he was hit by a car which failed to stop for police in Oadby, Leicestershire.
Days later, Holly Burke, an off-duty police community support officer, died after being seriously hurt in a crash in the West Midlands on Tuesday.
Her car was struck by an alleged hit-and-run driver who was being chased by police.
On the same day a 26-year-old refugee from Eritrea was killed after being hit by a police car which was responding to a 999 call in east London.
Officers stopped and gave first aid to the woman before an ambulance and air ambulance arrived. Paramedics fought to save her life but she was pronounced dead at the scene.
In the last of the cases, Jessie Whitehead, a 74-year-old, died in hospital in Bedworth, Warwickshire, after she was hit by a car that was responding to an emergency call.
She had been riding a mobility scooter at the time of the crash.
"The death of even one person on the roads, let alone four in such a short space of time, will be of concern," IOPC deputy director-general Ian Todd said.
"While there has been a spate of road traffic fatalities over the past week, deaths and serious injuries following pursuits and emergency responses are thankfully rare in comparison to the large number of such incidents police are involved in on a daily basis."
The police watchdog will look at the driving of the patrol cars during the investigations
The IOPC will also examine the crashes individually and look at the circumstances surrounding each death, including whether the police drivers were suitably trained.
"We will also look to see if there are changes and improvements which can be made to those procedures to reduce the risks of deaths and serious injuries in the future," Mr Todd added.
Additional reporting by agencies
