A man was killed and a woman was critically injured Friday night when their motorcycle hit an on-ramp wall, throwing them from an overpass.

The motorcycle remained on the on-ramp, Charleston police Lt. Jason Emanuele said. He said the accident happened at 7:41 p.m. on the overpass that connects the Crosstown to Interstate 26. The busy on-ramp also carries traffic headed from Charleston to the Arthur Ravenel Jr. Bridge to Mount Pleasant.

Emanuele, who is commander of the police departmentís special units, said investigators were told the motorcycle was trying to pass a car on the right when it struck the retaining wall on a sharp curve. The impact dislodged the riders from the motorcycle.

It appears that the riders fell about 40 feet.

Emanuele said a nurse who was driving on the on-ramp witnessed the accident and ran down the ramp to near King Street, and tried to help the fallen pair.

The man was driving the motorcycle, and the woman was a passenger, Emanuele said.

They landed near railroad tracks between King and Meeting streets.

No names were available. The woman was taken to Medical University Hospital in critical condition. No further information about her condition was available late Friday.

The on-ramp was not closed during the accident investigation, but the two right lanes were blocked.

Emanuele said that among the police departmentís traffic accident reconstruction team that will be conducting an in-depth probe is an officer who just returned Friday after taking a course this week focusing on reconstructing motorcycle accidents.

Neither of the accident victims was wearing a helmet, Emanuele said, adding that probably would have mattered little considering the distance they fell.

The fatal motorcycle accident is not the first for the on-ramp. In November 2009, William Heath Cox, 35, died after being tossed over the railing. He hit the wall also before falling, police said.