Costs also are up. The six-month estimated bill for traffic deaths, injuries and property damage is $152 billion—24% higher than 2014. The costs include wage and productivity losses, medical expenses, administrative expenses, employer costs and property damage.

Related: The 9 deadliest driving behaviors, by the numbers

"Follow the numbers: The trend we are seeing on our roadways is like a flashing red light–danger lies ahead," said Deborah A.P. Hersman, NSC's president and CEO. "Be a defensive driver and make safe decisions behind the wheel. Your life really depends on it."

What's to blame?

The high death and injury toll could be due to many factors; however, an improving economy with lower gas prices and unemployment rates have brought increases in vehicle miles traveled. According to the Energy Information Administration, average gas prices are 30% lower than they were in 2014 and are projected to remain relatively stable heading into 2016. This generally means an increase in traffic as more people can afford to drive, and many travel longer distances and take vacations.

Cellphone use may also be a culprit. An NSC study earlier this year indicated cellphone use is a factor in one quarter of all accidents.

Here are the 15 states with the highest percentage increase in traffic deaths from 2014 to 2015 (for the first six months of the year), according to NSC data.

Traffic backs up on Interstate 10 as authorities investigate the scene of a fatal accident on Sunday, July 26, 2015, in Phoenix. Multiple people were killed in the rollover crash and several were injured. (Alonso Parra via AP)

15. Arizona (tie with Colorado)

Traffic deaths up 18% over the first six months of 2014.

451 traffic deaths from January 1 through June 30, 2015.

A tour bus involved in an accident with a tractor trailer Friday, April 17, 2015, sits the foreground while the bus that was following sits behind the wrecked vehicle on the shoulder of the westbound lanes of Interstate 70 in Aurora, Colo. Officials say that 12 people were injured, two of them critically, in the four-vehicle wreck that shut down the highway for several hours during a spring storm packing heavy rains and fog. (AP Photo/David Zalubowski)

14. Colorado (tie with Arizona)

Traffic deaths up 18% over the first six months of 2014.

236 traffic deaths from January 1 through June 30, 2015.

A dump truck crash snarls traffic on the Inter County connector between Layhill Road and New Hampshire Avenue in Silver Spring, Md., Wednesday, May 13, 2015. (AP Photo/Jose Luis Magan)

13. Maryland (3-way tie with North Carolina and Utah)

Traffic deaths up 19% over the first six months of 2014.

224 traffic deaths from January 1 through June 30, 2015.

12. North Carolina (3-way tie with Maryland and Utah)

Traffic deaths up 19% over the first six months of 2014.

634 traffic deaths from January 1 through June 30, 2015.

11. Utah (3-way tie with Maryland and North Carolina)

Traffic deaths up 19% over the first six months of 2014.

129 traffic deaths from January 1 through June 30, 2015.

(Photo: Shutterstock.com)

10. Wisconsin

Traffic deaths up 19% over the first six months of 2014.

254 traffic deaths from January 1, through June 30, 2015.

This Saturday, May 2, 2015, photo provided by the Livemore, Calif., Police Department shows the scene after a car crashed into an apartment complex, killing a woman and toddler and slightly injuring two other children, in Livermore in Northern California. Police arrested Brian Jones, of Livermore on suspicion of gross vehicular manslaughter while intoxicated and another alcohol-related driving count, Officer Ryan Sanchez said. (Ryan Sanchez/Livermore Police Department via AP)

9. California (tie with Wisconsin)

Traffic deaths up 20% over the first six months of 2014.

1,566 traffic deaths from January 1 through June 30, 2015.

8. Louisiana (tie with California)

Traffic deaths up 20% over the first six months of 2014.

330 traffic deaths from January 1 through June 30, 2015.

(Photo: Kelly Nelson/Sutterstock.com)

7. South Carolina

Traffic deaths up 21% over the first six months of 2014.

445 traffic deaths from January 1 through June 30, 2015.

(Photo: Shutterstock.com)

6. North Dakota

Traffic deaths up 22% over the first six months of 2014.

61 traffic deaths from January 1 through June 30, 2015.

(Photo: Shutterstock.com)

5. Indiana

Traffic deaths up 23% over the first six months of 2014.

356 traffic deaths from January 1 through June 30, 2015.

(Photo: Chris Parypa Photography/Shutterstock.com)

4. Georgia (tie with Minnesota)

Traffic deaths up 26% over the first six months of 2014.

657 traffic deaths from January 1 through June 30, 2015.

3. Minnesota (tie with Georgia)

Traffic deaths up 26% over the first six months of 2014.

187 traffic deaths from January 1 through June 30, 2015.

In this March 30, 2015 file photo, workers pull a van out of a canal at the intersection of U.S. 27 and State Road 78 West, near Moore Haven, Fla. Federal transportation officials are investigating the crash that killed eight members of a Florida church who were riding in a type of van that has raised safety alarms for many years. (AP Photo/Luis M. Alvarez)

2. Florida

Traffic deaths up 29% over the first six months of 2014.

1,441 traffic deaths from January 1 through June 30, 2015.

This photo provided by the Oregon Department of Transportation shows the scene of an accident on I-84 in Oregon on Saturday, Jan. 17, 2015. Black ice is believed to be the cause of a freeway pileup involving more than a dozen tractor-trailers Saturday morning in eastern Oregon, police said. (AP Photo/Oregon Department of Transportation)

1. Oregon

Traffic deaths up 59% over the first six months of 2014.

204 traffic deaths from January 1 through June 30, 2015.

Safety tips

To help ensure safety, the NSC has the following recommendations for drivers:

• Make sure every passenger buckles up on every trip.
• Designate an alcohol and drug-free driver or arranging alternate transportation.
• Get plenty of sleep and taking regular breaks to avoid fatigue.
• Never use a cell phone behind the wheel, even hands-free.
• Stay engaged in teens' driving habits. Teens are three times as likely to crash as more experienced drivers.
• Learn about your vehicle's safety systems and how to use them. My Car Does What can help drivers understand the ins and outs of features such as adaptive cruise control, blind spot warning systems and backup cameras.

Related: They did what? Distracted driving survey reveals wide range of dangerous behaviors