You don’t have to drive far in a typical American town before you see it: A pictographic image of a child (implied to be a boy), perhaps chasing a ball, perhaps poised in midstride, perhaps atop a seesaw, perhaps with a jaunty cap, perhaps with a companion or parent. And then, some variation on these words: “Slow: Children at Play.” As a child, you may have played near such a sign. Maybe you made the callous jokes about “slow children” playing. You probably assumed they were there for a good reason; that they made your community a safer place for children; that their message, beaming with retro-reflective glory, was duly observed by drivers, who suddenly snapped to hyper-vigilant readiness at the wheel, scanning the road in a methodical, Terminator-like radar-sweep.

You would be wrong.

Despite the continued preponderance of “Children at Play” on streets across the land, it is no secret in the world of traffic engineering that “Children at Play” signs—termed, with subtle condescension, “advisory signs”—have been proven neither to change driver behavior nor to do anything to improve the safety of children in a traffic setting. The National Cooperative Highway Research Program, in its “Synthesis of Highway Practice No. 139,” sternly advises that “non-uniform signs such as “CAUTION—CHILDREN AT PLAY,” “SLOW—CHILDREN,” or similar legends should not be permitted on any roadway at any time.” Moreover, it warns that “the removal of any nonstandard signs should carry a high priority.”

There are several reasons engineers don’t like the signs. The first, and most simple, is that if you are driving in an area where children are actually playing, you will, it is hoped, notice them before you notice a sign warning you of them. Or, more to the point, that you will have noticed that you are driving in an area (say, a residential street) where there are likely to be children about. “I find it amazing that people think that a 30-in X 30-in square sign (that is only a little less than 6.25 square feet of sheeting material when you make the corners rounded) will make a difference in driver behavior,” one engineer complained on an Internet forum. “If the driver does not notice the characteristics of a neighborhood as they drive down the street, why would they notice a sign as they pass it, or remember it for more than a few seconds once they have passed it.”

Another problem is that, as with “Bridges Freeze Before Roadway” signs on a hot summer day, the many times a driver will drive down a street when no children are playing gradually condition him to disregard the warning, or even view it with a bit of suspicion. Who hasn’t gone past a rusting “Deaf Child” sign and wondered if the child hasn’t long since grown up or moved away?

The Federal Highway Administration, in its Manual of Uniform Traffic Control Devices, the sacred Ur-text of road signage, does not sanction “Children at Play” signs. It notes: “The use of warning signs should be kept to a minimum as the unnecessary use of warning signs tends to breed disrespect for all signs. In situations where the condition or activity is seasonal or temporary, the warning sign should be removed or covered when the condition or activity does not exist.”

There are other problems. Does the presence of a “Children at Play” sign subtly hint that there aren’t children at play in other locations? Does the sign breed a false sense of security? And does it, as some engineers suggest, encourage the idea that the street is to be used as a play area—which could, as one report suggests, expose municipalities to the possibility of tort liability?

If the sign is so disliked by the profession charged with maintaining order and safety on our streets, why do we seem to see so many of them? In a word: Parents. Talk to a town engineer, and you’ll often get the sense it’s easier to put up a sign than to explain to local residents why the sign shouldn’t be put up. (This official notes that “Children at Play” signs are the second-most-common question he’s asked about at town meetings.) Residents have also been known to put up their own signs, perhaps using the DIY instructions provided by eHow (which notes, in a baseless assertion typical of the whole discussion, that “Notifying these drivers there are children at play may reduce your child’s risk”). States and municipalities are also free to sanction their own signs (hence the rise of “autistic child” traffic signs).

Which is not to discount the concern of parents, or anyone else, about traffic speeds on residential streets (even if, as police have told me on more than one occasion, it’s often the very same residents who are the problem). In fact, as much as engineers know about how “Children at Play” signs do not work, one might argue there is much they do not know about how they work. As one report notes, “though these all discourage the use of such signs, none of them cites specific research demonstrating that these signs are ineffective.” In other words, even if the “Children at Play” signs are a placebo, that doesn’t mean they’ve been tested with the same rigor as a new drug.

One of the things that is known, thanks to peer-reviewed science, is that increased traffic speeds (and volumes) increase the risk of children’s injuries. But “Children at Play” signs are a symptom, rather than a cure—a sign of something larger that is out of whack, whether the lack of a pervasive safety culture in driving, a system that puts vehicular mobility ahead of neighborhood livability, or non-contextual street design. After all, it’s roads, not signs, that tell people how to drive. People clamoring for “Children at Play” signs are often living on residential streets that are inordinately wide, lacking any kind of calming obstacles (from trees to “bulb-outs”), perhaps having unnecessary center-line markings—three factors that will boost vehicle speed more than any sign will lower them.

It’s also not uncommon to see “Children at Play” signs in the presence of 35 mph speed-limit signs, which is roughly akin to trying to put out fire with gasoline. It’s not simply that fatality risks begin to soar at impact speeds of more than 20 mph, but that, as a study by John Wann and colleagues at Royal Holloway University in London has suggested, children, until well into their teens, are unable to detect during a normal crossing of the street the approaching speed and distance of cars above a threshold—also 20 mph. This study adds legitimacy to the increasingly popular idea, as introduced in the U.K. in 1991, that residential areas be designated as 20 mph zones. (Research by the Transport Research Laboratory has found, among other things, a 60 percent crash reduction during the “after” period in 20 mph zones.)

It is, of course, no secret that children are risky pedestrians. “Children are particularly vulnerable to pedestrian death because they are exposed to traffic threats that exceed their cognitive, developmental, behavioral, physical and sensory abilities,” reads a typical child safety document. “Children are impulsive and have difficulty judging speed, spatial relations, and distance.”

This is all true, and well and good, but it overlooks one thing: The same could be said about many adult drivers, the ones putting those children at risk. As is often the case in driving, when we meet the enemy, it is us. You want difficulty in judging spatial relations? Consider the research, by Dennis Shaffer, that showed people reporting 10-foot-long highway stripes to be two feet long. You want difficulty estimating speed? Consider this study, which found drivers underestimating their speed in the presence of children by upwards of 50 percent. You want exceeded sensory abilities? Consider the widespread phenomenon of “overdriving” one’s headlights. You want trouble estimating distance? Ask any driver how many feet they’ll need to stop, driving at 65 mph. You want impulsive? Who’s reaching across the seat for that buzzing BlackBerry? Driving, developmentally, turns us into children.

But it’s easier to blame the victim and to say the child trying to cross the road, on which cars were moving faster than they should, impulsively darted into traffic. As a paper in Injury Prevention noted, however, “drivers who hit a child pedestrian were more likely to have had a prior citation, more citations, more safety violations, a suspended or revoked license, or more negligent operator points than drivers who did not hit a child pedestrian in the study period.” Not to mention the many cases (to cite just one recent example) of children struck by drivers when they weren’t even on the road, when they weren’t “playing,” but simply walking.

“Children at Play” signs may not be doing any good, but simply removing them isn’t likely to do much more. What we need is lower speeds, and a more compelling way of achieving those speeds than signs that everyone has seen, and no one seems to mind.