Welsh Government figures show the number of driving tests taken in Wales has fallen 6,000 in the last year.
And almost a quarter of households don't own a car.
The area with the least number of car owners is Merthyr Tydfil and the highest Powys.
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There will be a fresher feel with temperatures falling lower than on recent nights.
The accident between junction 24, Coldra, and 23A, Magor, has closed the eastbound carriageway.
Eloise Parry, 21, died after taking diet pills containing the poisonous Dinitrophenol (DNP).