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The Straight Dope



It has been over ten years since the U.S. government mandated that all cars be equipped with a "center high-mounted stop lamp" or "CHMSL," as it is referred to in my vehicle's shop manual. The CHMSL, of course, is the little red brake light that is mounted in the rear window of a car to catch the attention of drivers who might not notice your side-mounted brake lights. Has this invention reduced the incidence of rear-end collisions? --Tom Meyer, Trenton, NJ

Believe it or not, there are people whose job it is to keep track of stuff like this. Every year the federal government publishes thousands of pages of regulations covering everything from auto emissions to the privatization of the National Helium Reserve in Amarillo, Texas. Business types have long complained that all this red tape drives up prices and, even worse, doesn't do any good. So the Government Performance and Results Act of 1993 and Executive Order 12866 now require federal agencies (such as the National Highway Traffic Safety Administration) to periodically review the effectiveness of their regulations (such as those requiring CHMSLs, commonly pronounced chim-sulls), with a view to getting rid of those that are useless or counterproductive. Too bad this rule doesn't apply to people, too.

A 1998 NHTSA review shows the limits of what regulation can do. CHMSLs have been required in autos since the 1986 model year and in light trucks since 1994, after pilot tests involving taxicab and corporate fleets found that the lights reduced rear-end collisions by 35 percent. Tests immediately after CHMSLs became mandatory showed lower but still substantial reductions--15 percent in one study, 11 percent in another. (The difference was partly accounted for by the fact that fleet-test data was reported directly by drivers, whereas postregulation data was derived from police reports.)

As time went on, though, the utility of CHMSLs continued to decline, reaching a "long-term effectiveness level" of 4.3 percent in 1989. NHTSA's rueful conclusion: Once the novelty wore off, most people stopped noticing CHMSLs and went back to their old habits. The lights are still considered cost-effective, though--the total annual cost of CHMSLs is about $206 million, and even at 4.3 percent they save an estimated $655 million in medical costs and property damage each year. Moral: Changing the rules is easy; changing human nature isn't. See for the NHTSA's report.

As a child I used to get excruciating growing pains in my legs--I would literally wake up crying. I always wondered what caused these pains and why they were only in my legs. Was it actually my leg bones growing that I felt? Curiously, during the biggest growth spurt of my life, nearly four inches in about a year from 17 to 18, I don't remember my legs hurting a lick. I gather the experts don't really know. One doctor said the pains were the result of injuries during the day that children at play overlook, but at night when they're relaxed the pain returns. That sounded like hooey to me. Please tell me you know something the experts don't. --J., Tampa, FL

Wish I could oblige, but research into growing pains has been minimal, a common state of affairs when a health complaint goes away on its own and doesn't seem to produce any long-term consequences. Here's what we know:

(1) Growing pains probably have little to do with growth. The pains are most common in children ages 4 to 12, whose growth rate is lower than that of both infants and adolescents; most growth occurs near the knees, but the pain is not centered there; children who have growing pains grow at the same rate as those who don't.

(2) Hypothesized causes of growing pains include (a) rheumatic fever (discredited), (b) anatomical problems such as discrepancy in leg length (discredited), (c) fatigue or minor injuries during the day (unconfirmed, although one study claims that muscle stretching significantly reduces the incidence of growing pains), and (d) emotional problems (controversial--some investigators say that children with growing pains are moodier and more prone to report other types of pain, suggesting GPs are psychosomatic, but critics have faulted these conclusions on various grounds).

(3) Research on growing pains has been hindered by the lack of adherence to a consistent definition for the condition. The most widely accepted criteria: (a) occurs late in the day or at night; (b) not joint related; (c) persists at least three months; (d) intermittent, with pain-free intervals lasting days to months; (e) pain severe enough to interfere with sleep; (f) no other clinical signs (what's called a "diagnosis of exclusion"). In practice this boils down to: If you can't figure out what else it might be, call it growing pains. Treatment: massage, stretching, analgesics, rest, heat. Sorry, but that's the best I can do.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.

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