The Met Office has issued a yellow warning of fog affecting Gloucestershire, South Gloucestershire, Bristol, Devon, Somerset, Dorset and Wiltshire.
It will come into force from 7pm tonight and last until 10am tomorrow, Tuesday 25 November.
Patchy fog is expected to form soon after dark tonight and then will become quite widespread and locally dense. It will freeze during the evening, reducing visibility to less than 100m in places.
It is likely to persist through tomorrow's rush hour but may begin to disperse as cloud increases from the south.
Transport could be disrupted so commuters should be prepared.
Police are appealing for people who may have seen a fatal crash in thick fog. It happened on the A3102 at Mile Elm at 9.40am when a woman Fiesta driver from Wiltshire hit a goods vehicle travelling in the opposite direction.
The driver died at the scene and her family are being informed.
Police would particularly like to speak to a van driver who arrived at the scene after the crash but left before emergency services got there.
Fog is affecting visibility in parts of Bristol, Bath and Somerset. If you are heading out, do take extra care.