Just as the summer riding season gets rolling, Michigan bicyclists are finding themselves on an increasingly dangerous road ahead.

An MLive analysis of Michigan State Police accident data shows fatalities involving bicyclists surged 57 percent in 2015, and the 33 deaths caused by the accidents mark the heaviest death toll in at least 18 years.

"People have got to learn to slow down, pay attention and share the road," said Paul Kissel, a veteran rider who constructed a "ghost bike," marking the location of one of 2015's fatalities in Muskegon County.

In that case, Susan Cummings, a nurse from Lake Orion, was with a large group of riders when she was struck from behind by a hit-and-run driver in August.

Ronnie Jo Claflin, a Muskegon Township man, faces a Tuesday, June 28 court date and potential 15-year felony for Cummings' death.

In another case, Michael Anthony Curley, 26, a Pittsfield Township father, was killed by a hit-and-run driver in Ann Arbor in November. Ahmed Abdullahi Mohamed, 25, entered a no contest plea at a probable cause hearing in the case earlier this year.

The fatalities totaled 33 -- the highest number since the MSP's Office of Highway Safety Planning first started publishing annual crash fact sheets, which break down accidents by type, in 1997.

Melody Kindraka, a spokeswoman for the Office of Highway Safety Planning, said traffic deaths overall, which had been trending downward in Michigan, jumped 10 percent from 2014 to 2015.

Alcohol-involved, bicyclist, teen and motorcycle deaths were all up more than 20 percent -- but the largest increase -- 57 percent -- was the category for bicyclist fatalities, and those numbers don't include five bicyclists who died after they were struck by a pickup truck north of Kalamazoo earlier this month.

"We have researchers working on" reasons why, examining the time of day the accidents occurred, under what weather conditions, and which parts of the state they happened, Kindraka said.

In April, her office promoted the state's first pedestrian and bicyclist safety conference, focusing on traffic dynamics, planning and engineering best practices as well as the presentation of a multi-year study of bicycle and pedestrian crashes in Michigan by researchers at Western Michigan University.

Among the findings of that study:

More than 40 percent of bike riders involved in accidents were on a sidewalk prior to the crash, suggesting a correlation between riding on the sidewalk and a failure by either the cyclist or the motor vehicle driver to yield or to disregard of a traffic signal or sign.

Approximately 93 percent of crashes occurred on streets without a dedicated bike lane.

The most common risk factors involved in bicyclist accidents were failing to yield or disregarding a traffic control (58 percent), motorist overtaking bicyclist or bicyclist overtaking motorist (27 percent), loss of control or turning error (6 percent), and bicyclists riding the wrong way (5 percent).

The Western study concluded that a lack of streets that accommodate bicyclists encourage riders to use sidewalks, where they are most prone to crashes involving failure to yield.

"As supported by the analysis, emphasis on implementation of education and enforcement countermeasures is vital in reducing the frequency and severity of pedestrian and bicycle crashes," the report also concludes.

One such effort has started in Grand Rapids, which adopted a "safe passing" law less than one year ago.

The law requires that motorists keep at least 5 feet between the right side of their vehicle and the bicycle they are passing, and the city also required bicycles for night riding to be equipped with a white light on the front and a red reflector or light on the rear.

"Generally we have not heard a lot of push-back from the rule, which some could say is surprising," said Suzanne Schulz, the city's managing director of design and development. "I thought it might be more vocal, but because ... we are trying to identify this with safety ... the community seems to be accepting of it."

Shultz said there's a recognition that crashes can be a result of false moves by either bicyclists or motorized vehicle drivers.

Bike riders can travel in the wrong direction, not wear clothing that's easily seen, not use reflectors or lights or ignore the rules of the road.

Motorists contribute to accidents by not looking before turning at an intersection, and "simply not paying attention," the managing director said.

Ron Fonger is a reporter for MLive. He can be reached at 810-347-9963 or rfonger1@mlive.com.