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The Emperor has decreed that the speed limit for horses and horse-drawn vehicles within city limits is to be 5 leagues per hour. As usual he has left the implementation details to his Scientific Adviser.

Using only medieval-type measuring devices, how can the Adviser satisfy the Emperor's wishes?

You can assume that there are wooden odometers available

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and medieval clocks

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It presumably doesn't make any difference to the answer but a league can be assumed to be 3 miles.

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    $\begingroup$ how would they properly monitor everyone? $\endgroup$ – Rose Dec 26 '18 at 0:44
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    $\begingroup$ Since engine(animal) is fixed maybe limits the wheel size and gear ratio. $\endgroup$ – user6760 Dec 26 '18 at 4:44
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    $\begingroup$ "Is this your own horse and cart, Sir ? Just a quick document check, nothing to worry about. When was the last time you quaffed ?" $\endgroup$ – StephenG Dec 29 '18 at 3:11

12 Answers 12

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No technology is needed at all:

From http://www.speedofanimals.com/animals/horse:

All horses move naturally with four basic gaits: the four-beat walk, which averages 6.4 kilometres per hour (4.0 mph); the two-beat trot or jog, which averages 13 to 19 kilometres per hour (8.1 to 12 mph) (faster for harness racing horses); and the leaping gaits known as the canter or lope (a three-beat gait that is 19 to 24 kilometres per hour (12 to 15 mph), and the gallop. The gallop averages 40 to 48 kilometres per hour (25 to 30 mph).

Only the gallop is over the limit: The speed limit can be enforced with a simple no-galloping rule. Any minimally-trained observer can tell the difference between the gaits.

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    $\begingroup$ @chaslyfromUK - if you are asking how the Scientific Advisor would arrive at a similar conclusion, a known distance, a minuteglass, and the cooperation of a few horses and riders on a nice day would be sufficient to gather the requisite data. The analysis is trivial unless you want to go the extra mile to determine statistical distributions of each gait. $\endgroup$ – user535733 Dec 25 '18 at 16:40
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    $\begingroup$ And, to be perfectly honest - in medieval times, the Emperor is far more likely to say "no horse may gallop within the city walls" instead of "no horse may exceed the speed of five leagues per hour" $\endgroup$ – Chronocidal Dec 25 '18 at 21:21
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    $\begingroup$ This additionally makes it easy for the riders or coachmen, who would generally lack a speedometer, to follow. $\endgroup$ – Jan Hudec Dec 26 '18 at 0:06
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    $\begingroup$ I wonder if medieval towns had No Wake Zones (like for boats); signs that say, e.g., Trotting Only. $\endgroup$ – Mazura Dec 26 '18 at 4:34
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    $\begingroup$ @Mazura - think about the characteristics of a medieval town: Narrow streets with lots of blind corners, Lots of people, trash, and sewage on many of those narrow streets. Only a fool would gallop in such a risky environment. Horses were expensive -- few such fools had the means to own horses. $\endgroup$ – user535733 Dec 26 '18 at 4:42
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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.

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    $\begingroup$ The noise, +1 $\endgroup$ – Mazura Dec 25 '18 at 20:18
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    $\begingroup$ Accepted answer talks about a 'No Galloping Rule'; how you would enforce that can be left up to the imagination. How you would know when to enforce it, is when a carriage going 30MPH down a cobblestone street sounds like it's destroying the pavement along its way. That way your guards can sit in a shack and play cards all day like they're supposed to. $\endgroup$ – Mazura Dec 26 '18 at 4:28
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    $\begingroup$ A cobblestone street has speedbumps every few inches. $\endgroup$ – Shawn V. Wilson Dec 26 '18 at 17:28
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    $\begingroup$ I read once that supposedly the Romans had speed bumps in ancient Rome: blocks about 6-8 inches tall spaced a few inches apart at intersections. They also doubled as out-of-water pedestrian crossing for when it rained. $\endgroup$ – Clockwork-Muse Dec 26 '18 at 18:03
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    $\begingroup$ Galloping horses (without a cart) do not have axels and are unlikely to be slowed by speed bumps. $\endgroup$ – James Jenkins Dec 27 '18 at 13:57
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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).

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    $\begingroup$ ‘Do you know how many Royal Horses you were doing back there, sir?’ $\endgroup$ – Joe Bloggs Dec 28 '18 at 12:52
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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.

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    $\begingroup$ Theoretically a speeding guy could run until a certain point and then stop before the measuring guy, fooling the system. $\endgroup$ – L.Dutch - Reinstate Monica Dec 25 '18 at 17:03
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    $\begingroup$ This is literally how "average speed cameras" work on the Motorways in the UK $\endgroup$ – Richard Dec 25 '18 at 19:30
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    $\begingroup$ @L.Dutch, they could, but stopping to ensure their average speed was low enough would defeat the point of running in the first place :) $\endgroup$ – Dancrumb Dec 25 '18 at 21:08
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    $\begingroup$ @Dancrumb, that's what some people do with modern systems: they run like hell between the measuring points, then stop for a coffee just to lower the average speed. I am not saying it is smart. $\endgroup$ – L.Dutch - Reinstate Monica Dec 26 '18 at 3:34
  • $\begingroup$ In medievel times, human life was cheap and wages were low. So there's the ability to throw manpower at the problem, and use a quantity of trained observers. $\endgroup$ – Criggie Dec 30 '18 at 13:26
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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.

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  • $\begingroup$ Usually cities put their clock on a tower in the middle so everyone can see it, there's no need to transport it around the city to get the time in each street. $\endgroup$ – Pete Kirkham Dec 27 '18 at 9:07
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    $\begingroup$ @PeteKirkham, medieval clocks had usually only the hour arm. $\endgroup$ – L.Dutch - Reinstate Monica Dec 27 '18 at 9:08
  • $\begingroup$ Only because they didn't need to measure anything to the second. $\endgroup$ – Pete Kirkham Dec 27 '18 at 9:10
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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.

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    $\begingroup$ Well, speaking historically, in medieval times no Emperor had and could have absolute authority. Best they could do is to control some tiny demesne, while subordinating rest to the vassals/governors. And those subordinates were not always zealously loyal. And there were not a lot of soldiers(professional standing army), most of the army was levies(drafted peasants and city-dwellers). So, probably, there were not a lot of soldiers to waste on watching some carts passing by. $\endgroup$ – user28434 Dec 26 '18 at 16:11
  • $\begingroup$ @user28434 The question does assume the Emperor cares enough to make it happen. If the Emperor throws enough money and personal attention at it, something will probably get done. My point is that it doesn't have to be done well, because anyone doing it in the Emperor's name is beyond criticism. $\endgroup$ – Graham Dec 26 '18 at 21:40
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A technically simple - but reasonably expensive to implement - solution is to add grooves to all the roads (running crosswise, not lengthwise). If they are spaced evenly, then the frequency of the noise made by a cart traveling along it corresponds to the speed the cart is traveling at. The rule can be as simple as prohibiting the carts from making a noise higher than a particular note, and can be checked by anyone with minimal training.

It will be expensive to add grooves to all the roads, though, including maintaining them when they wear out, and it will be unpopular to make noisy streets even noisier.

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    $\begingroup$ I wonder if the vibrations could be subsonic for legal speeds but audible for illegal ones. However the system won't work for horses being ridden by a single rider without a cart. $\endgroup$ – chasly from UK Dec 27 '18 at 12:30
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    $\begingroup$ Given medieval sanitation, they'll fill up pretty quickly with something. Urban roads had an open sewer running down the middle. $\endgroup$ – Criggie Dec 30 '18 at 13:27
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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.

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Major roads (anywhere where it would be possible to exceed the speed limit) will have a... something... alongside them traveling at exactly 5 leagues per hour. As long as you are going slower than the something, you are fine. If you are passing it, you're in trouble...so that part is simple.

Now the more complicated part is what should be the "something". I think a marble run could do the trick. You'll want to shield it somehow (something translucent like waxed paper) to prevent wind from interfering, and you'll need to create some friction in the track to maintain a constant (non-accelerating) speed. Also you'll want something like a watermill to power the marble lift to get the marbles back to the top again. It might take a really smart person to come up with the idea in that era, but actually building it is should be within medieval technology (although building one on every major road will be a big undertaking).

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  • $\begingroup$ I suppose you could simply have a stream with corks floating along as markers. You would have to pump the water to the other side of the street for the return journey. $\endgroup$ – chasly from UK Dec 27 '18 at 12:31
  • $\begingroup$ @chaslyfromUK I wasn't sure if you could get it 1) consistent enough, and 2) fast enough... I think (guess) you would still need roughly the same profile of drop followed by a slight down gradient with some kind of friction. $\endgroup$ – user3067860 Dec 27 '18 at 15:12
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An answer in what I think is the spirit of your question: Use the wooden odometer you have included in your question, and use it to haul a flag up a pole on the cart, via friction. Calibrate these things so that the flag slips down the driving element (say, twine) at a known rate such that if the cart goes faster than X speed, the twice drive the flag up the pole faster than it can slide down -- a lossy driven element. If the speed is below X, then the flag remains at the bottom of the pole, out of sight.

Then you local modder community can sell shifty apparati that decrease the friction, and your guards can inspect poles and twine and flags, all under the guidance of your science advisor.

I think this is what you're looking for based oion the way you have phrased your question.

Source: I did this in A.D. 1453, and the slow response time of shifting reinforcements around was part of why we lost Constantinople.

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  • $\begingroup$ I like this lateral (vertical ?) thinking - very ingenious - thanks :-) This is definitely worth pursuing and would make a good story element for characters wondering what the flag was for. Of course it only works for carts, not unharnessed horses unless everyone has to hire an odometer on entering the city - that would be a good source of revenue. $\endgroup$ – chasly from UK Dec 28 '18 at 9:57
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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.

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  • $\begingroup$ This works if this is an absolute-everywhere limit. But (a) the horses can go faster for a short distance - e.g., trotting at a normal speed and the driver sees something rolling down the street and speeds up to pass before it gets to the middle of the street (I think speeding up a horse/cart is going to be easier than braking to a stop (the horse can stop easily but carts didn't have antilock brakes) and (b) if they go out of town for a bit and forget to reset the odometer before coming into town then they would be caught yet not have broken the law. $\endgroup$ – manassehkatz Dec 25 '18 at 18:46
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Pressure plates a known distance apart in the road - one to open a water valve, and the other to shut it. The water thus pours into a graduated container, and the amount of water dispensed is inversely proportional to the speed of the animal or vehicle activating the plates. A technician is required to empty the container after each vehicle, and to intelligently resolve multiple-vehicle situations.

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  • $\begingroup$ I think the 'multiple-vehicle situation' is the real flaw in your answer. This is a busy city :-) $\endgroup$ – chasly from UK Dec 27 '18 at 21:55

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