It’s tough to make predictions,” Yogi Berra warned us. “Especially about the future.” That is a true statement concerning virtually every field of human endeavor, but it is especially true when it comes to predicting the future of cities and the way people will be moving around them many years hence. We love to make bold guesses about how we will transport ourselves a generation or two down the road. These guesses have one thing in common: They almost always turn out to be wrong.

Back in 1894, a distinguished panel of New York citizens peering into the urban future issued a distant early warning. By 1930 or so, they said, the streets of Manhattan would be virtually impassable due to an exponential increase in the amount of manure dropped by horses pulling carriages. That prediction probably made sense at the time. It just failed to account for the invention of the automobile.

Fifty years ago, there were scientists at the nation’s leading universities speculating that by the end of the century, Americans would be commuting to work in personal jet planes they could park on backyard landing strips. It didn’t seem far-fetched. We may laugh about it now, but “The Jetsons” looked to much of the mid-century engineering elite like a glimpse into an inevitable future. Once again, it was a bit off the mark.

Thirty years ago, no one in his right mind would have looked at the decrepit New York subway system and seen it as an engine of 21st-century affluence that would transform lowly Brooklyn into a bastion of million-dollar condominiums and high-technology entrepreneurship. But that one actually happened.

Today the Internet is awash in scenarios describing how driverless cars will change our cities and our lives in the space of just a few years. No doubt the driverless revolution will arrive someday. How soon is a question on which, given history, we have every right to be skeptical.

The fact is that when it comes to transportation, we love to make wild guesses about the future, even though our previous ones didn’t turn out right. And thanks to computers, we can generate large volumes of data in support of any given future we wish to promote. Some of this guesswork is harmless, but some of it can lead us to dubious public policy choices.

Consider, for example, the National Environmental Policy Act. NEPA was enacted in 1970, at a moment when environmental activists were giddily enthusiastic about their ability to produce a cleaner planet through federal regulation. Its focus was almost entirely on protecting Earth’s air and water and the creatures who share the planet with us. It wasn’t meant to be a statement of transportation policy, and for most of its early history it wasn’t that.

But the language of NEPA was so vague as to make the law useful for a wide variety of crusades far outside its original intentions. The law promised to foster “an enjoyable harmony between man and his environment” and “a wide sharing of life’s amenities.” Future legislation dealing with land use was required to carry with it an environmental impact statement if it constituted a “major federal action significantly affecting the quality of the human environment.” What exactly that meant was left to courts and the litigators who came before them.

Given the way the law was written, it was a relatively simple matter for activists to file suit against a new highway or bridge or public transportation system on the grounds that it would somehow interfere with the harmony between man and his environment. Thousands of such lawsuits were filed, and many of them were successful in slowing projects down, and sometimes knocking them out altogether.

For a long time, this didn’t bother me very much. Most of the environmental litigation that interfered with major highway projects, especially in congested cities, seemed to be at the very least preventing construction of an eyesore and much of the time striking a blow for urban survival and recovery.

But it soon became a common requirement for communities and developers planning any new transportation project to justify it by predicting how it might be used 10, 20 or 30 years later, even though no such prediction stood much chance of being accurate. Planners often had to thread the needle when it came to feigning clairvoyance. If the usage they forecast for a project was too heavy, the project could be halted on the grounds that it would lead to unacceptable congestion. If the forecast came in too low, a judge could declare it to be an unnecessary intrusion on the pristine land around it.

And so we come to 2017, and the strange case of Maryland’s Purple Line project. The Purple Line is (or would be) a 16-mile light rail system with 21 stations connecting communities in two huge suburban Washington, D.C., counties, Prince George’s and Montgomery. It would cost something over $2 billion to construct. It’s been a dream of local planners and urban advocates for more than 25 years. In 2014, the dream seemed to be coming true. The Federal Transit Administration (FTA) under President Barack Obama agreed to spend $900 million on the project -- enough, in combination with private and state and local government funding, to get it built. The first trains were projected to begin running in 2022.

In the summer of 2016, the Purple Line was five days away from getting its federal grant money. But at the last moment, a new player entered on the scene: U.S. District Judge Richard Leon. The judge issued a response to a NEPA lawsuit brought by some suburbanites living in the vicinity of the project. These residents had no legitimate environmental concerns; they just didn’t want a transit line in their neighborhoods. But they knew how to take advantage of the giant loophole that NEPA had become.

The plaintiffs managed to convince Leon that the Purple Line planners hadn’t done enough to project what the line’s ridership might be as far out as the year 2040. Actually, the planners had made some projections, but the judge said they weren’t good enough. He said they failed to account for declining ridership on Washington, D.C.’s Metro system. While Metro is basically unrelated to the Purple Line, it could be responsible for generating as much as a quarter of the new line’s passengers. So what if a weakened Metro, the judge wanted to know, left the Purple Line, 25 years from now, without a sufficient number of riders? Maybe, then, the whole project was unnecessary. Go back and make more guesses, Leon ordered.

The FTA took several months and then returned with essentially the same package of projections it had prepared several years earlier. The package offered five different scenarios, ranging from a substantial recovery in Metro ridership to a collapse of the Metro system entirely. Even if Metro ceased to exist, the agency estimated, there would still be plenty of riders in 2040 to justify the new project. And besides, what did all this have to do with protecting the environment? The answer, of course, was nothing.

In May, the judge answered back. The Purple Line planners hadn’t given him the data he wanted. He wasn’t releasing any money until they went back and gave him a whole new supplemental statement, with accurate numbers out into the distant future. The project managers pointed out that if the federal funding didn’t arrive by June, they would have to delay the whole project, and possibly cancel it altogether. Too bad, the judge essentially told them. That wasn’t his problem.

The FTA appealed Leon’s ruling, and late last month a federal court issued a temporary stay of the judge’s decision, while the case is being reheard. That allowed the state to resume work on the line, at least for now.

But the whole case provides glaring evidence of how years and millions of dollars can be wasted arguing about projections that can’t possibly be made with even a shred of confidence.

It could be that by 2040 Metro and the Purple Line system will have combined to spark thriving new development all along the new route. It’s also possible that by 2040 Metro will have fallen into disuse. But why stop there? Maybe by 2040 driverless cars will have pushed all forms of public transportation off the road. Maybe everyone will be teleporting to work. Maybe anything.

The point is that these things are not just uncertain, but unknowable. Predicting traffic patterns in Maryland in 2040 is about as valid as projecting the increase in horse manure in Manhattan between 1894 and 1930. Anybody who pretends to have precise information is either a fool or a self-interested charlatan.

But there’s a larger point to be made here. Whether or not to build the Purple Line is a question for the democratic process -- for the citizens we elect as legislators and appoint as managers. Reasonable people will differ on it. But when a judge hijacks the whole issue and issues rulings on spurious legal grounds, he undermines public trust in the judicial system.

In most recent election years, there have been moves in some states to curtail the power of judges, to make them answerable to the voters, usually by forcing them to stand for re-election more often. I’ve been against all these actions, on the grounds that judges need insulation from the vagaries of public opinion. Usually the efforts fail. But I have to admit something: When I read about decisions like the one involving the Purple Line, I begin to wonder if the anti-judge activists don’t have a bit of the truth on their side.