Everyone can understand the logic behind setting maximum speed limits. It’s obvious that there are speeds that are too high to be safe to drive on a given road. You have less time to react to what happens in front of you, more chance of losing control of your vehicle going around a curve, etc. Not to mention, all else being equal, the faster you drive the more fuel you use, at a time when we’d really like to be reducing our dependence on fossil fuels rather than using more of it.
But what of minimum speed limits? While maximum speed limit signs are ubiquitous, one will also occasionally see a sign specifying a minimum speed limit. Sometimes they’re specified on the same sign in fact, so for example a stretch of highway might have a sign posted with a 70 MPH maximum, and a 40 MPH minimum. What is the logic of requiring people to drive above some minimum speed?
For reasons both of safety and convenience, it is preferable that there not be too great a difference between the vehicles traveling the fastest and the vehicles traveling the slowest on a given road.
For safety’s sake, you want drivers to be able to anticipate a certain flow of traffic and settle into driving at that pace. Imagine, for example, a road where the speeds at which people are driving range from 60 MPH to 70 MPH, and another otherwise identical road where the speeds at which people are driving range from 15 MPH to 65 MPH. The average vehicle on the first road would be traveling considerably faster than the average vehicle on the second road, so if speed were all that mattered, the first road would be less safe.
But think about what it would be like to drive on the second road. People would be constantly shifting lanes, they would rarely be able to settle in and follow someone at the same pace for a significant stretch (since it’s unlikely they’d find themselves traveling behind someone moving at the same speed at which they want to move), and they would have trouble maintaining proper distance between vehicles (because they’d have to be constantly making new judgments as to how quickly they’re closing on the vehicle in front of them, since they’re all traveling at such different speeds).
So in fact the second road would be so chaotic and hard to navigate that despite the fact that the average speed would be lower, that would actually be the less safe of the two roads.
Then there’s the issue of convenience. You want people to be able to rely on getting where they’re going in a predictable about of time. You want them to know that if they take this highway between City A and City B 70 miles away, they’ll be able to get to City B in an hour or so.
Now sometimes that’s not possible, due to heavy traffic, bad weather, an accident or construction that’s closed some lanes, etc. But without minimum speed requirements, you’d be adding yet another factor that makes it unlikely people would be able to anticipate how long it’ll take them to get where they’re going. They wouldn’t know if today will be a day with a disproportionate number of 15 MPH and 20 MPH drivers on the road holding things up.
One more point: A minimum speed limit need not be posted for one to be in effect. By default, it is always a violation of traffic laws to travel at a speed that creates a significant safety hazard or nuisance to other drivers.
Again, this is more obvious when we think of it in terms of maximum speed limits. On icy, curvy roads in the dead of winter in the middle of the night, you aren’t legally allowed to drive 70 MPH even if that’s the posted maximum speed limit, because it would be patently unsafe to do so.
But think about the flip side of that. Let’s say the state legislature passes a law prohibiting people over the age of 50 from marrying flounders raised in captivity under 16 inches long. And the several hundred people over 50 who’d intended to marry flounders raised in captivity under 16 inches long angrily descend on the capital and proceed to protest the law by driving 1 MPH up and down the downtown streets all day.
They obviously aren’t violating the maximum speed limit, and there typically is no posted minimum speed limit on city streets, so are they abiding by the law?
A moment’s thought reveals that they are not, and why they are not. Think of how damaging it would be, how inconvenient it would be for other drivers, how the city’s economic activity would largely grind to a halt if this were allowed to go on all day (or all week or all month or however long the protestors decide to keep it up).
The logic of requiring certain minimum speeds to be maintained to avoid creating a nuisance like that is really akin to why you’re not allowed to simply park your car in the middle of the road and impede traffic. Traveling at a speed of 0 MPH is just a more extreme case of driving too slowly.