I am fairly certain they see a sign like that and simply think that it is to tell cyclists to keep out of their way
This is why we need more signs that say "cyclists use full lane"
Combined with lanes that are narrow enough that a driver really isn't going to feel comfortable going faster than 30km/h anyway! I don't want to "share the road" with vehicles that are going to be super aggro, fast and dangerous. But it's not bad when everyone is going closer to the same speed anyway.
I always do a sad laugh when I pass the "share the road" sign, almost as much as when I see the "single file. No passing." sign as motorists furiously close pass no matter what. 😣
Love when I'm riding at 30 in a 25 zone and cars roar past me because they can't accept being behind anybody but another car
