Fewer Americans died in traffic accidents in 2014 than in the previous year, making it the second year in a row where fatalities have decreased. However, the National Highway Traffic Safety Administration says preliminary data shows a large increase in deaths on the road in the first half of 2015.

Using information from its Fatality Analysis Reporting System, the NHTSA determined that 32,675 people died in motor vehicle accidents in the United States in 2014. This total marked a 0.1 percent decrease from 2013 and also brought the fatality rate to a record low of 1.07 deaths per 100 million miles traveled.

In the first six months of 2015, a total of 16,225 people were killed in traffic accidents. This estimate is 8.1 percent higher than the total from the first half of 2014, which had a total of 15,014 fatalities. The fatality rate also rose 4.4 percent to 1.06 per 100 million miles during this period.

The Federal Highway Administration has recorded year-over-year growth in travel for 19 consecutive months, including an estimated 3.5 percent increase in miles covered in the first half of 2015. In addition to the increase in travel, the NHTSA says the initial increase in fatalities for 2015 may be due to factors such as job growth, lower gas prices, and more driving by young adults and leisure travelers.

Overall traffic fatalities have been falling steadily in the past decade, with the exception of a 4 percent increase in 2012. The NHTSA says partial year estimates are volatile and likely to change, but that the increase is still a cause for concern.

"These numbers are a call to action," said Transportation Secretary Anthony Foxx. "Everyone with a responsibility for road safety—the federal, state, and local governments, law enforcement, vehicle manufacturers, safety advocates, and road users—needs to reassess our efforts to combat threats to safety. USDOT will redouble our efforts on road safety, and we expect our partners to do the same."

Most motor vehicle accidents did not involve either a death or injury. Of the 6.1 million accidents reported to police in 2014, 72 percent involved property damage only.

Fatalities have declined 25 percent since 2005, when 43,510 people were killed in motor vehicle accidents. Crash injuries fell 13 percent in the same period, to a total of 2.3 million in 2014.

The decline in traffic fatalities was smaller than last year, when the number of deaths on the road fell 3.1 percent between 2012 and 2013. However, the NHTSA notes that 2014 was the safest year on record for passenger vehicle occupants. A total of 21,022 people died in these vehicles, the lowest number since FARS data collection began in 1975.

During the year, 1,678 drivers between the ages of 16 and 20 died in crashes. There were also 581 people in this age range who were riding as a passenger with a young driver and killed in a crash.

A total of 726 cyclists were killed in collisions with motor vehicles, a decrease of 2.3 percent. There were 4,884 pedestrians who died in motor vehicle crashes during 2014, an increase of 3.1 percent from the previous year.

Many of the fatalities were preventable. Nearly half of the passenger vehicle occupants killed in a crash—49 percent—were not wearing a seat belt at the time.

Drunk driving crashes accounted for 9,967 deaths, or 31 percent of all fatalities. The share was higher in Connecticut, where 38 percent of the state's 248 traffic deaths in 2014 were a result of drunk driving. In Rhode Island, 35 percent of the state's 52 total fatalities were related to alcohol impairment.

Massachusetts, North Dakota, and Texas had the highest share of drunk driving fatalities. In each of these states, 41 percent of the total fatalities for 2014 were a result of intoxication.

The NHTSA says unhelmeted motorcyclist deaths were much more common in states that did not have strong helmet laws. Only 151 unhelmeted motorcyclists died in states with universal helmet laws, compared to 1,565 unhelmeted motorcyclists who died in states with more lenient rules.

Speed was a factor in 28 percent of fatalities, contributing to 9,262 deaths. Distracted driving was reported in one out of every 10 fatalities, or 3,179. At least 846 people died in drowsy driving accidents, equal to 2.6 percent of the total figure.

According to NHTSA research, human error is a critical cause of 94 percent of crashes. Vehicle issues are a critical cause of only 2 percent of accidents.

"Behavioral safety programs are the heart of NHTSA's safety mission," said NHTSA Administrator Mark Rosekind. "While great public attention is focused on safety defects and recalls, and rightfully so, it is time as a nation to reinvigorate the fight against drunk and drugged driving, distraction, and other risks that kill thousands every year, and time for state and local governments to reassess whether they are making the right policy choices to improve highway safety."

The NHTSA says it has recently launched several efforts to improve safety on the road, including initiatives to support enhanced safety features in vehicles. A series of regional meetings will focus on ways to combat unsafe driving behaviors such as drunk, drugged, distracted, and drowsy driving; speeding; and failing to use safety features such as seat belts and child seats. In addition, the NHTSA says it will focus on ways to protect more vulnerable road users such as pedestrians and cyclists.