Powys and Gwynedd have been named in the top ten of local authorities in the UK lacking mobile phone coverage on their roads.
Powys (411 miles) came second only to Highland local authority area in Scotland (910 miles).
Motorists in Gwynedd would be unable to call for help or look up directions on over 213 miles of the road network, the RAC Foundation said.
Across the UK, about 2% of roads have no mobile coverage at all.
A further 44,368 miles, 18% of all roads, have only partial voice coverage, with not all operators providing a signal.
Motorists who rely on their smartphones to access the internet for route planning and to check for congestion could get into difficulty on the 5,452 miles of road with a complete absence of 3G coverage.
In terms of 4G, just 51% of the road network has full coverage.
The research was based on analysis of data published by communications regulator Ofcom.
The RAC Foundation's Director, Steve Gooding, said that more needed to be done to improve connectivity in order to make roads safer and reduce congestion.