I agree with your sentiment and would like to take it a step further.
As someone who lives in San Francisco and commutes using all three of bike, foot, and car (and back in college rode a skateboard around a large and busy campus) I don't think skateboards belong on the sidewalk - and definitely not motor powered ones.
Anything that has the potential to move much faster than a person running at full speed is extremely dangerous on the sidewalk for everyone involved: cars, bikes, and especially pedestrians. It breaks everyone's mental model for what could possibly be coming from where. Cars making a right turn do not expect bikes or skateboards to entering the sidewalk at 5-10 mph from their blind spot and so they almost never check. Pedestrians turning a corner don't expect the possibility of something oncoming that is moving too fast to stop before hitting then and so they almost never look. And so on..
As an individual YOU might be a considerate skateboarder who has learned when and where to be careful. That's great, but it doesn't change the fact that there are still idiots and assholes out there that are going to do idiotic and asshole things. The only system that makes sense is: fast-stuff in one place where everyone knows to look out for fast-stuff, and slow-stuff on the sidewalk.
I don't think you can make rules based on potential speed, and your division is far too simplistic and arbitrary in terms of speed. While I agree my skateboarding is limited to 5-10 MPH (jogging-running speed), is that really that fast?
If I (and my skateboarding kind) should be in the street based on speed or potential speed, then so should rollerbladers, scooters, segways, motorized wheelchairs, anyone able-bodied person (as they have the potential to start running).
It's a complex issue and courtesy and being alert go a long way... as well as knowing the law. Wherever I'm supposed to be by law (NYC law leans toward the sidewalk) is where I need to be, and other people need to respect that.
Yup, its definitely a dicey issue. 5-10 mph on a skateboard is definitely a courteous and safe speed.. The problem is that 12-20 mph is also attainable and unfortunately not everyone is as courteous as you. It's the bad apples that spoil the bunch.
Yes, I do extend this to rollerbladers and scooters. Segues have a maximum speed of 12.5 mph and restrict their own downhill movement. I'd say they're sidewalk safe. Same with wheelchairs.
If there was a way to limit the speed of the board based on the grade of the hill, that could be an easy, innovative solution. I imagine that might be built in already to some degree.
As someone who lives in San Francisco and commutes using all three of bike, foot, and car (and back in college rode a skateboard around a large and busy campus) I don't think skateboards belong on the sidewalk - and definitely not motor powered ones.
Anything that has the potential to move much faster than a person running at full speed is extremely dangerous on the sidewalk for everyone involved: cars, bikes, and especially pedestrians. It breaks everyone's mental model for what could possibly be coming from where. Cars making a right turn do not expect bikes or skateboards to entering the sidewalk at 5-10 mph from their blind spot and so they almost never check. Pedestrians turning a corner don't expect the possibility of something oncoming that is moving too fast to stop before hitting then and so they almost never look. And so on..
As an individual YOU might be a considerate skateboarder who has learned when and where to be careful. That's great, but it doesn't change the fact that there are still idiots and assholes out there that are going to do idiotic and asshole things. The only system that makes sense is: fast-stuff in one place where everyone knows to look out for fast-stuff, and slow-stuff on the sidewalk.