As cities change towards a more human and sustainable perspective, governments include traffic regulations and special traffic signs like the following:

I took the photo in Pamplona, but you can see it in lots of cities. It means no entry streets except for bikes, which do can ride in the two ways. Of course, riders should put extra attention to the vehicles and people they come across when moving against car traffic. In fact, this sign is presented at the beginning of a pedestrian area in which vehicles different from bikes run like fire trucks, police cars, ambulances or delivery vans.