Hi Phil, I'm assuming there is at least one alternative route for the motorists to take. Well, in my opinion there isn't any reason not to consider a single lane bridge. As long as it meet the standards and warrants. On a past trip to northeast Italy, where I purposely decided to ignore the main roads and chose the local ones instead. I came across several one lane bridges, over small rivers, creeks and canals. I suspect these bridges serve the local community more, than they do the general motorist or tourist. They mainly fall under 2 types:-
Controlled by signage; Speed, height, width and weight limits. The give way sign, of opposing directional small red and large black arrow.Although I did see one which an opposing directional red and black car.
Controlled by signals; Only saw one of these. It still had the same signage but also there was a radio controlled, 2 aspect signal. On approach it was illuminated as Red, I was near to a stop when it changed to Green.
The signalised bridge did provide me with greater reassurance overall, being a foreign motorist. So I'm in agreement with Peter that signalised one lane bridge is the way to go.
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