A recent study by researchers at the University of Michigan Transportation Research Institute found that seniors are increasingly likely to hold a driver's license, while fewer people under the age of 69 are licensed.

Michael Sivak and Brandon Schoettle looked at driver licensure rates in the United States between 1983 and 2014 to determine how these rates have changed. The researchers found that the percentage of people under the age of 45 without a license has decreased continuously since the early 1980s.

The share of drivers between the age of 45 and 69 increased until 2008, at which point it decreased continuously. The percentage of people over the age of 70 who held a driver's license increased between 1983 and 2011 before declining.

"Overall, the future evolution of these changes will have potentially major implications for future transportation and its consequences," said Sivak. "Specifically, licensing changes will likely affect the future amount and nature of transportation, transportation mode selected, vehicles purchased, the safety of travel, and the environmental consequences of travel."

The researchers found that the decline in licensure was particularly noticeable among teenagers. Forty-six percent of 16-year-olds in the United States had a driver's license in 1983, but this share fell to 24 percent in 2014. In the same period, the licensure rate dropped from 69 percent to 45 percent among 17-year-olds, 80 percent to 60 percent among 18-year-olds, and 87 percent to 69 percent among 19-year-olds.

Among adults between the ages of 20 and 24, the share of Americans with a driver's license stood at 91.8 percent in 1983. This share was down to 82 percent in 2008, 79.7 percent in 2011, and 76.7 percent in 2014.

Sivak and Schoettle found that the licensure rate also dropped for people in their 30s and 40s. Between 1983 and 2014, it fell eight percentage points among those in their 30s and three percentage points among those in their 40s.

The share of people between the ages of 45 and 69 with a license increased in the 25 years between 1983 and 2008, rising from 83.8 percent to 95.9 percent. However, it dropped to 92.7 percent in 2011 and 92.1 percent in 2014.

People in their late 50s were more likely to drive in recent years. The licensure rate of the 55-59 age group was 91.8 percent in 2014, up from 88.2 percent in 1983. However, the 2014 share was still a drop of two percentage points from 2008.

There was a large increase in the percentage of adults over the age of 70 who were licensed. The share rose from 55 percent in 1983 to 78.4 percent in 2008 and 79.2 percent in 2011. It fell slightly in 2014, standing at 79 percent.

Richard Read, writing for the automotive site The Car Connection, says other studies have also suggested a drop in licensure rates among young adults in the U.S. A 2012 analysis by the Frontier Group found that the average annual mileage covered by drivers between the ages of 16 and 34 was 7,600 in 2009, down from 10,300 miles in 2001.

A 2013 study by the AAA Foundation for Traffic Safety found that only 44 percent of teenagers got a license in the first 12 months after they reached the age of eligibility. Forty-four percent said they didn't get a license because they didn't have a car, while 39 percent said they could get around without driving and 36 percent said they thought gas was too expensive. Twenty-nine percent said they were simply not interested in driving.

Read suggests that drivers over the age of 70 may be more interested in staying on the road since they grew up when vehicle ownership was more common. He says medical advances should also help seniors retain the ability to drive.

Other trends may be affecting younger adults' willingness to get a license. These include the restrictions set by graduated driver licensing, the ability to connect with friends through social media, the cost of owning a vehicle, the availability of public transportation and ride sharing services, and a desire to reduce one's impact on the environment.