The defendant had sought to keep other drivers from talking on their phones.

Cell phone jammers are illegal, and can draw large fines from the FCC, but people keep using them anyway: to keep workplace employees off the phone; to limit calls to and from a cosmetology school or sheriff’s office; or for peace and quiet on the bus.

Today’s offender, one Jason R. Humphreys, drove his daily commute along Interstate 4 between Seffner and Tampa, Florida with a jammer concealed behind the seat cover of the passenger seat. His reason? To keep people from talking on their cell phones while driving. But his chosen method not only blocked drivers’ calls – including those to 911 – but also calls by their passengers, people on nearby buses, and everybody else in range of his device.

When a local cell company reported receiving interference, the FCC’s Tampa office swung into action. Tracking down interference from a moving car is a lot tougher than finding one that stays still, but Mr. Humphreys’s signal was strong enough to locate – and indeed, was strong enough to shut down the sheriff’s deputies’ radios as they approached his SUV. When the FCC later tested the jammer, they found it clobbered not only cell phone frequencies, but many others as well, including some of those used for communications among first responders.

The FCC has tentatively found that Mr. Humphreys violated three separate rules: unauthorized operation of a radio transmitter, use of an illegal device, and intentionally causing interference. The base fines for these offenses are $10,000, $5,000, and $7,000, respectively. The FCC has discretion to raise them up to $16,000, and did so for each of the three violations, for a total of $48,000. The FCC could have, but did not, assess these amounts per day of a continuing violation, up to $112,500 for each violation. Still, unless Mr. Humphreys has a lot more resources than most of us, shelling out that $48,000 is going to put a dent in his spending plans.

We entirely understand Mr. Humphreys’ urge to take matters into his own hands; we likewise feel helpless and frustrated at drivers’ paying more attention to their cell phone conversations than to the traffic. But even so, a jammer can do more harm than good. And it gets very expensive, if the FCC tracks you down.