The District is finished with the warnings: Starting Thursday, drivers spotted making U-turns in the city's bike lanes will face a $100 fine.

Ahead of the crackdown, more than a dozen police officers, hack inspectors and transportation officials spent about two hours Wednesday along Pennsylvania Avenue Northwest dispensing warnings and fliers about the new penalties.

"The U-turns aren't allowed anymore for bicycle safety," said Jim Sebastian, a transportation planner for the D.C. government. "If you look at the data, we had about 16 crashes over a year on Pennsylvania Avenue, and 11 of them were related to U-turns. And those were just the reported crashes and doesn't count the close calls."

Shane Farthing, executive director of the Washington Area Bicyclist Association, said the new restrictions are critical to promoting safety in the designated lanes.

"This matters because it's a fundamental safety issue," Farthing said. "This is happening with enough frequency that there needs to be a show of force and some education that the city put these in and expects them to be used appropriately, just like any other piece of infrastructure."

One of the District's most visible thoroughfares, Pennsylvania Avenue is also the only city street with dedicated bike lanes in the middle of the road. Other bike lanes, like those along 15th Street Northwest, run along the curb.

But as the city's bicycle use has intensified, so have the accidents. In November, Mayor Vincent Gray ordered a rule change and said the long-standing policy of barring U-turns

in bike lanes could be enforced even when cyclists aren't nearby.

Although the new restrictions took effect on Dec. 1, authorities have been issuing citations only for blatant cases. Police officials said they had issued about 50 formal warnings ahead of Wednesday's final grace period.

The District's taxicab regulators said they will also play a role in enforcing the new rules because cabbies are barred from "any willful act which endangers or is against the best interest, health or safety of the passenger or public."

AAA Mid-Atlantic said it supports safe environments for cyclists, but the auto club also urged the District to enforce traffic laws against cyclists.

"I hope they will also be enforcing red lights and stop signs for bicyclists and that this will be equal-opportunity enforcement," spokesman Lon Anderson said. "To target cars for crossing bike lanes and not targeting bicyclists for crossing intersections at red lights seems to me to be hypocritical."