Officers were called at 3.55pm on Monday to the roundabout of Hertingfordbury Road and Cross Lane to reports of a collision involving a grey Mini Cooper and a grey Citroen Cactus.
The driver and passenger of the Mini have been taken to hospital.
Anyone who witnessed the collision has any dash cam footage or any information which could assist the investigation can report information by calling the non-emergency number 101 quoting ISR 336 of 19 September.
Alternatively you can contact Crimestoppers anonymously on 0800 555 111 or visit their website www.crimestoppers-uk.org.
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