I assume you mean for crossing busy roads and highways, not city streets? Certainly there are places for that, like Chicago's Lake Shore Drive. They're expensive and take a lot of space themselves, though, especially to meet accessibility requirements, and they can add considerable distance to the pedestrian's route.

Otherwise, what you're describing sounds like someone looked at this well-circulated cartoon and said "hey that's a great idea! Just raise the planks by 15 feet!" https://www.vox.com/xpress/2014/11/18/7236471/cars-pedestria...