The parkway was closed in both directions while police scoured the area for the man, who was armed with a shotgun or rifle, said Greg Shipley, a Maryland State Police spokesman.
The man walked up to a Jeep operated by a speed camera contractor late Wednesday morning, Shipley said. When he tapped on the rear window with the gun, the contractor became startled and began honking the Jeep's horn. When the man walked to the front of the vehicle and cracked the windshield with a hammer while yelling incoherently, the contractor got out of the Jeep and crouched near the guardrail until the man walked back into the woods, Shipley said.
Investigators don't know who the armed man is or what motivated the attack, state police Detective Sgt. Stewart Cumbo said. He did not know of any attacks like this one and said there does not appear to be any link to three incendiary packages sent to federal and Maryland government officials earlier this year. The packages, which ignited and burned the fingers of some state workers, contained messages condemning highway signs that urge motorists to report suspicious activity.
About 50 officers were searching for the gunman, described as a white man in his 60s with gray hair, wearing a red and blue plaid shirt and blue jeans. Police were using dogs and helicopters in the search.
There was no evidence that shots were fired, and no one was injured, Shipley said.
The encounter occurred near a construction site not far from the interchange between the parkway and Interstate 195, about two miles from Baltimore-Washington International Thurgood Marshall Airport.
Traffic was gridlocked in the area, and people traveling to the airport were advised to allow extra time and use either I-95 or I-97, said Charlie Gischlar, a State Highway Administration spokesman.
State highway crews are in the midst of a lengthy project to widen the parkway in both directions between I-195, the access highway for the airport, and I-695, the Baltimore Beltway. Speed cameras are authorized in the work zone, as they are in other highway work zones in Maryland. Signs alert drivers to the presence of the cameras.
Construction workers were at the site when the attack occurred, but work was shut down for the day, according to SHA spokeswoman Kellie Boulware. Construction would continue as usual Thursday.
A spokesman for Dallas-based contractor Affiliated Computer Services referred all questions to police.
The Baltimore-Washington Parkway is a 29-mile, four-lane divided highway connecting Baltimore city with the northeast corner of the District of Columbia that opened for traffic in 1954. The southern portion of the road is maintained by the National Park Service, while the northern portion is state-maintained and is also known as Maryland Route 295.
The parkway reopened shortly before 3 p.m.
Associated Press writer Ben Nuckols in Washington contributed to this report.