In trace_attributes (edges) what is the difference between speed and speed_limit

[DigNZ]

From looking at a sample of data - speed is more oftem populated that speed_limit which is sometimes 0. They are not always the same as each other for a given edge.

I'm working under the assumption that speed = average speed and speed_limit = posted speed limit, however as they are often the same this doesn't seem like that should be average speed.