I like Glorfindel's proposal, but the hourglass for every traffic warden is not really necessary within city limits as long as there can be a bell tower.

• Get a precise map of the streets, using the odometer cart.
• Label all intersections and intervals within longer streets.
• By law, all horsemen and cart drivers must carry a log book. At each sound of the bell, they must write down where they are. There are spot checks and serious penalties for inaccuracy.
• Every night the logs are collected and a random sample is analyzed.

The problem with that is that it only provides average speeds. If the average speed is above the limit then the top speed must have been above the limit as well. But an average speed below the limit does not prove that the cart never went above top speed.

#A Rube Goldberg Speedometer

1. An odometer drops stone balls onto a scale depending on distance traveled.
2. A mechanism triggered by the falling level of a water clock kicks stone balls of the scale at the right rate for top speed. If there is no stone, nothing happens. (That's where I get a little fuzzy. Ask your resident mad genius.)
3. Whenever there are two or more stones on the scale at the same time, the balance arm moves and breaks a seal.

Completely insane, of course.

I like Glorfindel's proposal, but the hourglass for every traffic warden is not really necessary within city limits as long as there can be a bell tower.

• Get a precise map of the streets, using the odometer cart.
• Label all intersections and intervals within longer streets.
• By law, all horsemen and cart drivers must carry a log book. At each sound of the bell, they must write down where they are. There are spot checks and serious penalties for inaccuracy.
• Every night the logs are collected and a random sample is analyzed.

The problem with that is that it only provides average speeds. If the average speed is above the limit then the top speed must have been above the limit as well. But an average speed below the limit does not prove that the cart never went above top speed.

A Rube Goldberg Speedometer

1. An odometer drops stone balls onto a scale depending on distance traveled.
2. A mechanism triggered by the falling level of a water clock kicks stone balls of the scale at the right rate for top speed. If there is no stone, nothing happens. (That's where I get a little fuzzy. Ask your resident mad genius.)
3. Whenever there are two or more stones on the scale at the same time, the balance arm moves and breaks a seal.

Completely insane, of course.