Boston drivers can be excused for being a tad skittish about falling objects, after experiencing two motorist fatalities in 2006: Michael Tsan Ty, killed by scaffolding that collapsed as he drove on Boylston Street in April, and Milena Del Valle, crushed by a falling ceiling tile in a Big Dig tunnel.

Now, concerns are being raised after a “replacement worker” — one of many who are in the midst of changing signage on some 2000 billboards in eastern Massachusetts and southern New Hampshire — fell from a South Boston billboard this past week.

The regular union workers have been on strike since March, when Clear Channel Outdoor Advertising implemented new employment conditions.

In addition to complaints about wages and benefits, the union workers — Local 391 of the Painters and Allied Trades District Council 35 — claim that Clear Channel Outdoor dissolved a joint labor-management safety committee. The company also switched the workers from hourly pay to piece-work — which, the union argues, rewards haste over safety.

After the union went on strike, Clear Channel brought in scabs, whose training has since been called into question.

In fact, Mayor Thomas Menino sent a letter to the company in May, expressing concern about safety issues posed by the replacement workers. “These billboards, which are often located on top of tall buildings and other structures, will be looked after by untrained, out-of-state workers,” Menino wrote. “The public must be able to rely on a well-trained, professional workforce.” The Boston City Council also passed a resolution expressing similar fears.

This past Tuesday, those concerns seemed prophetic, when one of the replacement workers fell while working on a billboard at 8 Silver Street, and was transported to Boston Medical Center.

A Clear Channel spokesperson confirms the incident, telling the Phoenix that the injured employee had completed an OSHA General Industry course, “as well as specialized training in climbing and working from elevations.” The worker fell from a ladder and suffered only minor injuries, according to the Clear Channel spokesperson. A union representative, however, says that witnesses describe a 30-foot fall from the billboard’s catwalk.

State labor groups are now calling for answers and action. Steve Murphy, chair of the City Council’s public-safety committee, says he intends to call for a hearing, and already has the support of council president Maureen Feeney and Bill Linehan, who represents South Boston, to do so.