Trung Huynh used a marked crosswalk with flashing yellow lights when he rode his bike across busy, six-lane Park Boulevard in Pinellas Park, Fla., one morning in June.

The 18-year-old didn’t make it to the median.

A white Chevy Malibu going an estimated 45 mph slammed into him and his bike, police said. Mr. Huynh died at the scene.

The collision added to the already-high cyclist death toll in Pinellas County. Its per-capita cyclist death rate for the past decade ranks No. 1 among the four counties in the Tampa Bay metro area, which has the highest fatality rate of any major metro area in the U.S., according to federal data.

The Gulf Coast region stands out in a state that itself stands out: Florida has by far the highest per-capita bicyclist death rate in the country.

Where Death Rates are Highest Bicyclist deaths from 2007-16 per 100,000 people by metro area* Fewer than 2 2 to 3.9 4 to 5.9 6 or more *Metro areas with at least 100,000 people and one bicyclist death Source: Analysis of National Highway Traffic Safety Administration bicycle death records

The number of cyclists killed in motor-vehicle crashes nationwide hit 840 in 2016—the most recent data available—according to the National Highway Traffic Safety Administration. That was the most since 1991 and a 35% jump from 2010.

A range of likely reasons explains the rise in deaths, including more overall vehicular traffic and driver distractions, according to people who track transportation trends.

Texting by drivers remains a big problem, said Deborah Hersman , chief executive of the nonprofit National Safety Council. “Almost every state in the country has a texting ban, but we still find drivers are texting behind the wheel,” she said.

Alcohol is a factor. In 2015, 22% of fatally injured cyclists, and 12% of drivers in these crashes, had a blood-alcohol content level of at least 0.08, the legal limit for motorists in most states, according to the nonprofit Governors Highway Safety Association.

Bicyclist deaths are rising... 3.0 deaths per 1 million people 2.6 2016: 2.5 2.0 1.5 ’14 ’10 ’15 ’16 ’09 ’12 ’13 ’08 ’11 2007 ...while the popularity of bike-sharing programs has exploded. 40 million bike-share rides 34.6 million 2017: 30 20 10 0 ’13 ’14 ’15 ’16 ’17 ’11 ’12 2010

The growing toll has come as bike-share programs in many U.S. cities have taken off. More than 75 communities now have bike-share programs, with the biggest in New York, Chicago and Washington, D.C. At least one international study found a rise in such programs is associated with a decrease in bike accidents.

While cyclist death rates have risen in many states since 2010, the three with the most fatalities since then—Florida, California and Texas—account for about 40% of all cyclist deaths, according to NHTSA, despite having 27% of the nation’s population.

Florida’s numbers are bad even when compared with other warm-weather states. Its recent 10-year cyclist fatality rate was 6.2 deaths per 100,000 residents; that is 59% higher than the rate in Louisiana, the state with the second-highest level.

Florida’s population is older and more densely packed, and the state gets a steady influx of tourists unfamiliar with local roads, said transportation-safety consultant Pam Fischer, who wrote last year’s GHSA report, which examined bike-safety issues nationwide. “You kind of mush it all together, and it helps us explain as best we can what’s going on out there,” she said.

Car-centric suburban development dating back decades is also a factor, say bicycle activists and transportation officials. Advocates for more bicycle-friendly roads say riders often must choose between riding on the sidewalk or bearing with cars whizzing by within inches of them.

Many suburbs are connected by six-lane arterial roadways that often have a speed limit of 45 mph or higher, said Ken McLeod, policy director at the League of American Bicyclists in Washington, D.C.

“If you want to go anywhere on a bicycle outside of your neighborhood, you have to go on that high-speed roadway because there’s no alternative,” Mr. McLeod said.

On the Road Though local streets are the most common type of road in the U.S., bigger thruways with higher speed limits tend to be more deadly for cyclists. The percentage of fatal crashes by road class: Common Crashes Class 2, 3 (non-limited access primary, secondary) 63.1% Right turn (perpendicular): Right turn (parallel): Typical class 4 road U.S., state or county highways and arterial roads connecting towns and neighborhoods; often have both a route number and a local name. The motorist doesn’t yield to the cyclist, who is not turning right. The motorist hits the cyclist, who is crossing from the sidewalk. Typical speed limit: 35-55 mph Typical class 3 road Class 4 (local) Non-arterial roads used for local traffic with a single lane in each direction, including city streets and small rural roads. 33.6% Typical speed limit: 25 mph Left turn: Unexpected move: The motorist doesn’t see the cyclist riding through the intersection. Other The cyclist enters the road at an unexpected location or angle. Interstate highways, ramps, alleys and other road types 3.3% Common Crashes Class 2, 3 (non- limited access primary, secondary) 63.1% Right turn (perpendicular): Right turn (parallel): Typical class 4 road U.S., state or county highways and arterial roads connecting towns and neighborhoods; often have both a route number and a local name. The motorist hits the cyclist, who is crossing from the sidewalk. The motorist doesn’t yield to the cyclist, who is not turning right. Typical class 3 road Typical speed limit: 35-55 mph Class 4 (local) Non-arterial roads used for local traffic with a single lane in each direction, including city streets and small rural roads. 33.6% Left turn: Unexpected move: Typical speed limit: 25 mph The motorist doesn’t see the cyclist riding through the intersection. The cyclist enters the road at an unexpected location. Other Interstate highways, ramps, alleys and other road types 3.3% Common Crashes Class 2, 3 (non-limited access primary, secondary) 63.1% Right turn (perpendicular): Right turn (parallel): Typical class 4 road U.S., state or county highways and arterial roads connecting towns and neighborhoods; often have both a route number and a local name. The motorist hits the cyclist, who is crossing from the sidewalk. The motorist doesn’t yield to the cyclist, who is not turning right. Typical speed limit: 35-55 mph Typical class 3 road Class 4 (local) Non-arterial roads used for local traffic with a single lane in each direction, including city streets and small rural roads. 33.6% Typical speed limit: 25 mph Left turn: Unexpected move: The motorist doesn’t see the cyclist riding through the intersection. The cyclist enters the road at an unexpected location or angle. Other Interstate highways, ramps, alleys and other road types 3.3% Class 2, 3 (non-limited access primary, secondary) 63.1% U.S., state or county highways and arterial roads connecting towns and neighborhoods; often have both a route number and a local name. Typical speed limit: 35-55 mph Class 4 (local) Non-arterial roads used for local traffic with a single lane in each direction, including city streets and small rural roads. 33.6% Typical speed limit: 25 mph Other Interstate highways, ramps, alleys and other road types 3.3% Common Crashes Right turn (perpendicular): Right turn (parallel): Typical class 4 road The motorist hits the cyclist, who is crossing from the sidewalk. The motorist doesn’t yield to the cyclist, who is not turning right. Typical class 3 road Left turn: Unexpected move: The motorist doesn’t see the cyclist riding through the intersection. The cyclist enters the road at an unexpected location or angle.

Still, some cycling advocates in Florida say cycling’s popularity is growing in the state. “If anything, more people are talking about where they want to ride in Florida than where they don’t want to ride in Florida,” said Becky Afonso, executive director of the Florida Bicycle Association.

State transportation officials say they have made a concerted effort since 2014 to boost cycling safety. The Florida Department of Transportation changed its standard width for bike lanes from 4 to 7 feet and now recommends buffered or protected bike lanes. The agency launched a $100 million push in 2016 to better light 2,500 locations where the number of nighttime crashes involving pedestrians and bicyclists was high.

Trung Huynh was killed crossing Park Boulevard in Pinellas Park, Fla., in June 2018. Photo: Huynh family

The state agency also lowered speed limits in some places, officials said. Local police have stepped up education efforts on topics such as using lights at night and riding with the flow of traffic.

“We are making some progress,” said Trenda McPherson, who manages the department’s bicycle and pedestrian safety program.

Cycling deaths in Florida fell to 116 in 2017, the fewest since 2010 and a significant drop from recent years, according to preliminary state data. But so far this year, bike fatalities involving motor-vehicles are trending higher: Through Sept. 23, the state said 95 cyclists had been killed in such accidents, putting it on track for about 130 for the year.

Six of those bicyclists died in Pinellas County, which includes the coastal cities of St. Petersburg and Clearwater.

“We are trying to really put in an all-points press in dealing with this issue,” said Whit Blanton, executive director of Forward Pinellas, the county’s land-use and transportation planning agency.

He blames a state growth-management law for the county’s large number of big, wide roads. “The laws have changed, but the infrastructure that was built is going to take a long time to change,” Mr. Blanton said.

Road Hazard A 6-mile stretch of Park Boulevard in Pinellas County, Fla., is the scene of several deaths and dozens of injuries incurred by cyclists, making it one of the most dangerous roads for bikes in the U.S. Below is a sampling of the circumstances around some of the deaths and injuries in the past decade, according to law enforcement. Crashes involving cyclist injuries Crashes involving cyclist fatalities Pinellas County 5:05 a.m., April 26, 2008 Victim’s age: 22 Cause: The cyclist was crossing Park Boulevard at a red light outside of the crosswalk and did not see the vehicle that struck her. 7:45 a.m., Dec. 31, 2014 Victim’s age: 32 Cause: The cyclist was on the sidewalk, crossing a driveway, when the motorist turned right and struck her. The motorist said she was reading a bible. 3:55 p.m., Sept. 28, 2015 Victim’s age: 55 Cause: In cloudy conditions, the cyclist crossed Park Boulevard into the path of the vehicle and was struck. 4:05 p.m., Sept. 12, 2011 Victim’s age: 18 Cause: The cyclist was riding on a sidewalk, crossing a driveway when the driver, exiting the driveway, lost control of the vehicle. 11:36 a.m., June 25, 2018 Victim’s age: 18 Cause: The motorist traveling at an estimated 45 m.p.h. failed to stop for the cyclist in a marked crosswalk with flashing lights. 3:37 p.m., November 25, 2009 Victim’s age: 65 Cause: The cyclist was riding on the sidewalk and ran into a vehicle turning onto Park Boulevard. Sources: Florida Highway Patrol records from 2011-June 2018 (injury and fatality locations); Florida Department of Highway Safety and Motor Vehicles (crash circumstances) Note: Locations for two Park Boulevard deaths in 2008 and 2009 are from National Highway Traffic Safety Administration records.

Since 2009, the county has added more than 60 miles of bike lanes, despite pushback from motorists. Colorado-based nonprofit People for Bikes this year gave St. Petersburg a 3.8 out of 5 score for its plans to improve the local biking infrastructure, one of the highest scores nationally.

The state transportation department recently conducted a study of Park Boulevard, one of the county’s most perilous thoroughfares. Officials found a majority of the cyclists ride on sidewalks rather than on the road, which doesn’t have bike lanes.

Many bike crashes occur when cyclists don’t use a crosswalk, a department spokeswoman said. The department plans by next summer to install three mid-block crosswalks on Park Boulevard featuring a red light that pedestrians or cyclists can activate.

Pinellas County officials said they plan to install this type of signal at the intersection where Mr. Huynh was killed. Construction is scheduled to begin by early 2019.

Rob Angell, deputy chief of operations of the Pinellas Park Fire Department, said the upgrade can’t come soon enough. The yellow-blinking-light crosswalk there now gives pedestrians and cyclists a false sense of security, he said. There is no guarantee drivers will stop, he said, and cars go “flying through there.”

Write to Scott Calvert at Scott.Calvert@wsj.com and Max Rust at Max.Rust@wsj.com