The U.S. transportation safety board is urging regulators to make side guards mandatory on new trucks to help prevent cars, pedestrians and cyclists from hurtling under the bone-crushing haulers – a move that could put pressure on Canada to act.

The side-guard recommendation and other proposals aimed at reducing injuries and deaths on American roads are before the National Highway Traffic Safety Administration (NTSB). A rule-making process on strengthening rear truck guards began this month, while an evaluation of side guards is continuing.

Required on most trucks in Europe and Japan, side guards cover the potentially hazardous gap between wheels. For cyclists, pedestrians and car passengers, they can be life savers. Side guards help deflect people away from trucks, lowering the risk of walkers and bikers falling underneath a truck in a crash and getting trampled by the rear wheels, as happened in Montreal recently.

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The death of teacher Mathilde Blais and the serious injuries suffered by a second cyclist drove home the dangers of riding near trucks without the guards. Both cyclists fell under the gaping side and were crushed in separate collisions about a week apart this spring.

Ms. Blais, 33, died instantly. The other cyclist, also a 33-year-old woman, was critically injured. A Montreal police spokeswoman said her condition has improved, but she is expected to remain in hospital, undergoing rehabilitation, for another three to four months.

Their crashes have reignited calls for safer trucks and roads in a city choked with congestion.

Saint-Laurent borough Mayor Alan DeSousa said a slew of measures is needed to reduce traffic deaths and injuries, including equipping trucks with guards. He has previously written to federal and provincial transportation ministers, urging them to move forward with side-guard regulations. (Transport Canada oversees new trucks; provinces have authority over existing haulers.)

Ontario's chief coroner also called for a national side-guard regulation after a review of cyclist and pedestrian fatalities last year.

"We need leadership on this issue," Mr. DeSousa said. Side guards "can save lives and save injuries."

Saint-Laurent is one of a handful of communities that has begun covering the side gaps of municipal vehicles. Mr. DeSousa said retrofitting costs about $1,500 per truck.

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"To do it on a voluntary basis would be almost a guarantee that it does not happen," he added. "Even one life saved is worth it, particularly since the costs involved are minimal."

What happens in the United States could shift Canada's position on the safety merits of side guards. The guards are not mandatory in either country.

Transport Canada has steadfastly rejected calls for side guards, saying its studies don't support making them mandatory.

"We do not intend to move forward with a regulation mandating side guards for newly built trucks and trailers at this time," Maryse Durette, a spokeswoman for the regulator, said in an e-mail.

In 2012, Transport Canada scrapped the second phase of a study evaluating whether side skirts, attached to trucks to reduce fuel costs, could also prevent cyclists from getting crushed under the big rigs. The regulator said it cancelled further testing because it had found no research to show truck skirts, which are lighter and more aerodynamic than guards, could make streets safer.

Yet according to the study's first phase, obtained by The Globe and Mail through access to information legislation in 2013, National Research Council testing of side skirts showed promise.

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Ms. Durette said Transport Canada is watching how its U.S. counterpart responds to the NTSB side-guard recommendation, noting "it will inform options in Canada."

Transportation officials from the two countries meet semi-annually to discuss common issues and expected regulatory changes. With thousands of trucks streaming across the border daily, the countries have pledged to harmonize trucking regulations.

The U.S. NTSB's recommendations, released in April, include addressing blind spots on tractor-trailers, equipping new trailers with side guards and strengthening rear guards. Canada's rules on rear guards are stronger, requiring the guards absorb greater force.

Transportation safety board spokesman Keith Holloway said the agency believes side guards that are designed to prevent injuries and deaths to car passengers would also benefit pedestrians, bicyclists and motorcyclists. The U.S. Truck Safety Coalition has also petitioned Washington for safer trucks.

In its review of truck guards, the safety board notes that in 759 pedestrian fatalities and 181 cyclist deaths involving trucks from 2005 to 2009 in the United States, 29 per cent of pedestrians and 55 per cent of cyclists collided with the side of trucks. Crashes were more common on the right side, where a trucker's blind spot is greatest.