mobile speed bump
n. A car that travels at the speed limit to force the cars behind to do the same.
The city of Vancouver is exploring the idea of sanctioning a grass-roots traffic-calming program that enlists the silent majority of reasonable, rational, law-abiding drivers to stop being so silent.

Instead they'd slap NEIGHBORHOOD PACE CAR stickers on their vehicles and set a highly visible example by rolling down Vancouver streets at lawful speeds.

Yes, you heard right: Making Vancouver streets safer simply by making a public point of driving the posted speed limit.
—Scott Hewitt, “How'd you like to be a mobile speed bump?,” The Columbian (Vancouver, WA), November 27, 2003
Elaine Clegg of Idaho Smart Growth, which runs Boise's Neighborhood Pace Car program, calls herself "a mobile speed bump." She slows cars driving behind her by driving the speed limit.

A sense of humor is required, she says. The Idaho program has bumper stickers saying "Follow me to the next red light," "Would you rather I were a speed bump?" and "Honk if you want me to slow down." So far, about 500 residents have joined the Boise program by signing a pledge and putting Pace Car stickers on their cars.
—Patrick McMahon, “Residents fight back against speeders,” USA Today, May 13, 2002
2001 (earliest)
Want to slow down traffic on your street?

Try a leisurely drive during rush hour. Boise, Idaho, residents did that through their PACE CAR program. They turned their cars into "mobile speed bumps" by purposefully driving the speed limit, forcing trailing drivers to ease off the accelerator.
—Toni Coleman, “Seminar to focus on 'calming' traffic,” Saint Paul Pioneer Press (Minnesota), May 14, 2001
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