Bike trails are not a substitute for a road. They generally have slower speeds (15 mph limits) and can often be more dangerous if they are multiuse trails.
There is actually a large movement among cyclists AGAINST bike trails and bikelanes for this very reason -- that it gives motorists all the more "reasoning" to tell them to get off the road.
This is unfortunate because I think bike trails are a good thing but they are not a substitute for riding on the roads and cyclists should NEVER feel they can't ride on a road because a trail is nearby.
I drive a car and pass cyclists all the time. I have no idea how it could be considered frustrating or dangerous. It's quite simple and once you're done passing you quickly catch up to right where you would have been.
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If once a man indulges himself in murder, very soon he comes to think little of robbing; and from robbing he comes next to drinking and Sabbath-breaking, and from that to incivility and procrastination.
- Thomas De Quincey