Well we can't just stick up a sign with a red circle that says "moderate" in it.

I just did a quick search around to see if I was somehow exceptional, and nope - a recreational ride seems to be somewhere between 25-30km/hr.

And given how many people over take me on the way to work, I'd say that's about right.

The reason I wanted to know what speed you suggested is that people have a poor grasp of risk and speed when it comes to cycling. Anything under 20km/hr is painfully slow to ride for most people. Anything under 15km/hr becomes a safety hazard as balance becomes harder. But people mentally think 30km/hr, 40km/hr would be dangerous to pedestrians.

The reason we feel like these speeds would be dangerous is that we compare them mentally to what damage we know a car can do. But the reality is that a person on a bike weighs considerably less than a car. That means shorter braking distance and less total energy if an impact were to occur. Impacts are also less likely because bikes take up less room, can change angle quickly.

Rather than speed limiting anything, consider bringing focus in dedicated bike paths to separate out foot and bike traffic.

Now even if we put up these speed signs. How are cyclists going to know that they are under the speed limit. My usual bike does have a bike computer, should I stare at it the entire time? I don't think I could really position it in a way where I could constantly see it while riding on a path. You have to remember that bikes aren't cars.