A yellow be prepared warning for snow and rain is in place for much of mid and south Wales.
The Met Office issued the yellow weather warning for snow for 11 of Wales’ 22 counties until 10:00 GMT.
A second yellow warning for rain covers 13 counties and is in place until 23:45 on Thursday.
Traffic analysts Inrix said stranded vehicles and heavy snowfall meant some roads in Powys, Ceredigion and Neath Port Talbot were impassable.
No school closures have been reported.
It said snow was completely covering all the lanes on the A465 Heads of the Valleys road between the Glynneath interchange and Cefn Coed roundabout in Neath Port Talbot.
It warned of hazardous driving conditions and said drivers should approach with care.
Vehicles are stranded along the A4109 between Seven Sisters and Banwen in Neath Port Talbot and the road is impassable, the analyst said.
It said the A482 in Lampeter, Ceredigion, was partially blocked by snow and stranded vehicles.
The A4221 between Abercrave and Coelbren in Powys is also impassable following heavy snowfall.
The weather warning for snow covers Blaenau Gwent, Caerphilly, Carmarthenshire, Ceredigion, Merthyr Tydfil, Monmouthshire, Neath Port Talbot, Powys, Rhondda Cynon Taff, Swansea and Torfaen.
The rain warning is in place in Blaenau Gwent, Bridgend, Caerphilly, Cardiff, Merthyr Tydfil, Monmouthshire, Neath Port Talbot, Newport, Powys, Rhondda Cynon Taff, Swansea, Torfaen and Vale of Glamorgan.
On Wednesday a multi-vehicle accident and snow forced road agencies to close a 12 mile (19km) stretch of the A470 between Merthyr Tydfil and Libanus in Powys in both directions.
South Wales Police said it was “receiving a high number of calls about weather-related incidents” and advised people to allow extra time if travelling.
BBC Wales weatherman Derek Brockway said conditions meant Wales was experiencing “heavy rain, sleet and snow”.