How steep can ramps be?

I'm curious to know, just how steep can ramps actually be? Is there a specific angle or gradient that's considered SAFE and practical for use in various applications, such as accessibility for wheelchairs or loading docks for trucks? Are there any regulations or standards that govern the steepness of ramps in different industries or contexts? I'm looking for a comprehensive understanding of the factors that influence ramp steepness and how they're balanced to ensure both safety and efficiency.