Turns out I wasn't imagining things - Nantes really IS the city with the worst traffic jams in France! According to a recent study by TomTom France, 42.3% of Nantes' streets are jammed every day - more so than even Paris, which comes in at 31.2%. (photo taken from Tuesday's Ouest France):
TomTom has been collecting data from its 5.5 million users, who drove over 2.87 million kilometers during the two year collection time period. They consider a road to be backed-up if the cars on it are circulating at less than 70% of the speed limit.

Not that this is likely to lessen the traffic issues there at all....but at least my putain's and various other swearwords will be justified!