An Ashburn teenager who was mountain biking on the Washington & Old Dominion trail yesterday morning was killed when he rode through a stop sign into traffic and hit a truck, Loudoun County authorities said.

Ryan Bickel, 17, was heading west near the western Loudoun town of Hamilton just before 10 a.m. when he rode into the trail's intersection with Ivandale Road, striking the passenger side of a Ford F350 truck. Bickel, who was not wearing a helmet, was pronounced dead at the scene, sheriff's spokesman Kraig Troxell said.

It was the second fatal cycling accident in two days within a half-mile section of the trail. On Wednesday, Sterling resident Eugenio Lucini, 57, rode from the trail into Route 704, a busy thoroughfare near Hamilton, apparently without yielding or stopping. The driver of an Oldsmobile traveling north braked and swerved but was unable to avoid striking Lucini, Troxell said. Lucini, who was wearing a helmet, died that day at Inova Fairfax Hospital.

Bickel would have begun his senior year at Broad Run High School on Monday. Reached by phone yesterday, family members declined to talk about the accident.

Wayde Byard, a Loudoun County schools spokesman, said Bickel was a "solid citizen" with a sharp sense of humor that made him popular with peers and teachers.

Grief counselors will be at Broad Run to help students cope, he said.

"It's . . . something you never want to see happen," Byard said.

Troxell said warnings are painted on the trail about 50 yards from Ivandale, and a sign at the intersection instructs bikers to stop. Cyclists often "treat stop signs as suggestions," Troxell said, even though stopping is required by state law. No other serious accidents have occurred on the trail in Loudoun, he said.

That section of the trail is heavily wooded, surrounded by a rural area. Even though the tree line stops before intersecting roads to give bikers time to see approaching vehicles, Troxell said, the dense foliage and the quiet can lull cyclists into letting their guards down.

"It can be deceiving," Troxell said. "You think . . . you're in the middle of nowhere, when in fact there's a lot of traffic on a lot of these back roads."

Staff writer Rosalind S. Helderman contributed to this report.