Used vehicle sales were more common in the United States in 2015, according to a study by the automotive site Edmunds.com. The study also found that former leases were a key factor resulting in record high used vehicle prices.

In its 2015 Used Vehicle Market Report, Edmunds.com tallied up a total of 38.28 million used vehicle sales. These sales included 11.41 million made through a franchise dealer, of which 2.55 million—or 22.4 percent—were certified pre-owned vehicles.

Certified pre-owned vehicles are typically younger and have lower mileage. They are also in good condition, subject to inspection and refurbishment before they are offered for sale, and may still be covered by a manufacturer's warranty. These vehicles were often leased by a previous owner, and thus driven for only a three-year period and kept within a mileage limit.

The report also found that drivers who buy certified pre-owned vehicles are similar to those who purchase new vehicles. Seventy-one percent of those who bought a certified pre-owned vehicle were 34 years old or older, the same share as new vehicle buyers. Among used car buyers, 42 percent were under the age of 34.

The report says the used car sales total by franchise dealers was a record for a single year, while the overall sales were at their highest point since 2007. In that year, a total of 38.28 million used vehicles were sold in the United States.

Used car sales were up 5.6 percent from 2014. In addition, franchise dealers' used car sales were up 2.1 percent and certified pre-owned sales climbed 9 percent.

The average price for a used car sold in 2015 hit a record high of $18,600. This price was up from the previous records of $17,700 in 2014 and $17,000 in 2013. The report suggests that this trend is a result of newer used cars and higher demand for older vehicles as well as pickup trucks and SUVs.

"The key factor driving all of the trends in used car sales today is the popularity of leasing, which is bringing younger and high quality used cars back to the market," said Jessica Caldwell, director of industry analysis at Edmunds.com. "We're truly in the midst of a Golden Age for CPO and near-new used cars. And with a record number of lease terminations expected in 2016, for the foreseeable future there certainly will be no shortage of supply to meet the growing demand for used cars."

The report notes that there was a large increase in leases between 2012 and 2013, with a 25.6 percent increase from 2.6 million to 3.2 million. As their three-year terms expire, many of them are returned to the dealer to be resold as used cars.

Leases have continued to be popular in recent years. They grew 11 percent to 3.6 million in 2014 and 12 percent to 4 million in 2015.

Fifty-four percent of the used cars sold in 2015 were three years old or younger. The average age for a used car was 4.4 years, down from 4.6 years in 2014.

The report says lease returns are one of the most common sources of used cars. It predicts that lease returns will increase by 26 percent this year, which in turn will result in more certified pre-owned sales.

However, the high volume of lease returns can also have an impact on prices. Between 2014 and 2015, the increase in average used car prices ranged from 2 percent on one-year-old vehicles to 19 percent on 11-year-old vehicles. By contrast, the oversaturation of three-year-old cars led to an average price drop of 4 percent among these vehicles.

Six-year-old vehicles were the only other age bracket that saw their typical price decrease, falling 2 percent between 2014 and 2015. The report says six years is the average age of a trade-in vehicle.

With gas prices remaining low, the greatest used vehicle price increases occurred in the pickup truck and SUV segments. The average price for a used pickup truck increased 9.9 percent between 2014 and 2015, while the average used SUV price grew 5.6 percent. By contrast, the typical price for a used luxury vehicle only increased by 0.1 percent.

Trade-ins are another major source of used car inventory, and 47 percent of new vehicle purchases involve a driver who trades in their former vehicle. But while lease end vehicles generally have little variation in their volume, condition, and mileage, these factors vary widely among trade-ins.