Most of the biking rules are simply good common sense: Don't go too fast. Stay to the right unless passing. Ride single file.
But there's a lesser known rule that has some bicyclists' warning bells going off.
Bikes cruising along the towpath are supposed to have a bell, a horn or some other noise-making device to warn people as they come up from behind.
Park rangers have been stopping bicyclists to check their equipment and hand them a copy of the rules. No one has been cited, he said.
"We'd prefer to keep it that way," he said.
Potomac Pushbikes helps park rangers publicize the rules, said employee Josh Vanderford.
Many bicyclists are surprised to learn they need a bell.
"They just kind of laugh about it even while they're handing us the money to buy one," he said.
But an informal survey on the towpath showed that people aren't having problems sharing the towpath, whatever mode of transportation they use.
"Everybody's really nice and courteous," said Debra Brewer, 36, of Hagerstown, who had just finished walking eight miles on Saturday.
Viki Abiba, 49, who recently moved to Williamsport, said the towpath near her home is much more peaceful than sections she used to frequent near Bruns-wick, Md., and Harpers Ferry, W.Va.
"It's a total mass collision of bicyclists," she said, and riders don't seem to be as considerate of the rules.
Most bicyclists said they respect the rule.
"I can see it when it gets crowded. More and more people are discovering the towpath," said Jeff Cooper, 34, of Hagerstown, whose bicycle was pulling a trailer with two small children.
Krissy Mabry, 27, of Crofton, Md., was riding her bike without a bell or a horn on Saturday.
But she maintains she isn't a nuisance. She always tries to warn people walking that she's coming up behind them.
She'll shout, "On the left," or squeeze her squeaky hand brakes.
"I don't try to scare `em at all,'' Mabry said. "I try to give them full awareness."
A group of nine friends from the suburbs of Washington, D.C., were bicycling the length of the towpath this weekend.
The all-male group, ages 16 to 21, left Cumberland, Md., yesterday with sleeping bags and tents.
Only the lead cyclist had a bell.
They didn't have a problem with the rule. The harder one to follow: The C & O's 15-mph speed limit.