With medieval roads? Don't even bother! Their axles will shatter if they try going too fast.

If you insist, put up speedbumps every hundred metres or so, or have twisted roads within your city limits. That will limit speed without needing the excess costs of constant monitoring. Anybody going too fast will either lose their wheels or make a sufficient racket to get bystanders' attention.

If you want a reliable measuring equipment, you need to have better than medieval clocks and odometers.

A clock which can be transported on bumpy medieval roads and still measure the time in a good way is probably out of time for the middle age.

I think it is more practical to limit the horse gait. Step, trott and gallop come with peculiar velocities and can be easily recognized without any measuring device. Furthermore, it's harder to tamper with them.

Frame Challenge

My knee-jerk reaction is that no one during medieval times would think in terms of distance-per-time. They're far more likely to think in terms of the behavior of the primary engine of motivation: the horse. In other words, the law would be that no one can run their horse above a canter. Everyone would understand that. Nobody would understand distance-per-time, even if you could invoke the tech to do it.

Add to this that historically politicians have never understood science and it's pretty much a guarantee that what you're seeking will never come to pass. The emperor would declare that no one can go faster than his horse (because a royal horse is the correct speed, after all).

I would just post an officer with an hourglass at one end of a street of known length. If he sees a horse or a vehicle passing a certain point at the beginning of the street, he turns the hourglass; if the road user passes the end of the street before the hourglass has finished, that means they're speeding, and eligible for a fine. If you make the fine high enough, e.g. seizure of the horse or carriage, people will make sure they'll never be speeding and keep a safe margin away from the maximum speed.

I'm not sure how precise medieval hourglasses were, but (sorry for using another system) your maximum speed amounts to 6.7 m/s; given a street of 100 meter, this is 14.9 seconds; you'll need 1.5 second precision (from both the hourglass and the observer) to obtain 0.5 league per hour precision.

Odometers/speedometers/ etc. will not work here because they can be mandated for carts but not for horses - you can make a speedometer today (and even 100 years ago) that could fit on a horse without weighing it down too much, but you could not have done that 500 years ago. So the only solutions are, as suggested by others, are to measure the speed externally by:

• Limit based on horse gait (if it comes through the gate with the wrong gait...) By far the simplest with the catch that someone might try to train their horse to run differently, though I think that would be impractical at best. This works as long as the desired speed matches well with different gaits, and is the simplest yet also subjective. But then again, the word of the emperor's traffic enforcer is considered trustworthy by definition.




• Hourglass. The good part about an hourglass in medieval times is that if you make one and it turns out to run too slow or too fast, due to the hole between the sections being hard to reproduce exactly the same between hourglasses, it really doesn't matter. The emperor & his scientific advisor produce one reference hourglass. When a new hourglass is manufactured, you add or remove sand until it runs for the same amount of time.

The emperor has another trick up his royal sleeve: If he wants to arrest someone, he can have his traffic enforcer release dogs into the street as his rival crosses the starting line. The horses react and gallop along uncontrollably for a block and the enforcer gets to write his ticket. Which can include, at his discretion, a visit to the dungeon.

No technology needed

These are medieval times. The Emperor's authority is absolute, and the Emperor's officers' authority can only be countermanded by the Emperor.

So you just need to show the traffic enforcement soldiers (because they will be soldiers) what a horse running at 5 leagues per hour looks like, and then say "anything faster than that, book them."

As with Judge Dredd, these soldiers are judge and jury. You have no right of appeal, no right of complaint, no right to do anything except pay them, which you do at the point of a sword. It doesn't matter whether they're right.

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.