Decades after the first "share the road" signs popped up, Maryland drivers could be fined as much as $500 if they pass within three feet of a bicyclist.

The law, which takes effect Friday, sets out to better define the rules for drivers and riders on the increasingly crowded roads, where close encounters, crashes and fatalities have become a reality of the dance between cars and bicycles.

Nationally, 630 cyclists died and 51,000 were injured in collisions with cars last year. Eleven bike riders died on Maryland roads last year, an increase over 2008, as more recreational riders and cycling commuters are encouraged by new bike lanes and increasingly frustrating automobile commutes.

Cyclists who ride regularly in Maryland said their dangerous encounters are caused by two types of drivers: those who are distracted and don't see them, and those who are angry and aggressive. Eli Hengst, a D.C. resident who often rides in Maryland, says it's not hard to tell who's who.

"It's actually pretty easy: The distracted drivers don't look back. They just keep on talking on their cellphone, swerving down the road," Hengst said. "The intentional acts are almost always accompanied by a middle finger, yelling of profanities as they drive by, honking of the horn or extreme quick jerks in a rider's direction as they try to ride you off the road."

Hengst said he experiences the unintended incidents almost daily and the angry encounters about twice a week.

Although the new law is intended to protect cyclists, it does mandate that they handle their bikes responsibly. They are required to maintain a steady course, stay to the right and use a bike lane, if there is one, or the shoulder, if it is smoothly paved.

Drivers who are about to enter or cross a designated bike lane or the shoulder are required to yield to cyclists.

The law might be a challenge for police to enforce, and the majority of the citations might be issued when a driver who violates the three-foot buffer causes an accident.

"I hope the new law will raise awareness - that is the key," said Jodi Grant, who bikes to work from her home in Bethesda to an office near the White House. "And if there are visible [crackdowns] maybe drivers will think twice. If drivers and bikers begin reporting those who violate the law and there are consequences, then it can have an impact."

Mark Smith said Annapolis police tracked down and gave a warning to a driver after this incident:

"On a narrow one-way city street in Annapolis a pickup truck driver put the 'squeeze play' on me and tagged me with his protruding truck mirrors," Smith recalled. "The mirror hit me in the left shoulder . . . but I was able to stay upright. Upon realizing what the truck driver did, he sped away and ran a red light to get away from the scene. I actually caught up to him several minutes later at a stoplight, and he would not make eye contact. I motioned for him to pull over, and he flipped me the bird and sped off, albeit not before I wrote down his tag number and reported him."