More than 45 million vehicles that were the subject of safety recalls — issued between 2013 and 2015 — are still not repaired, according to data from the National Highway Traffic Safety Administration (NHTSA) that was analyzed by J.D. Power.

Over the past 20 years, more than 437 million vehicles have been affected by safety recall decisions in the U.S. In 2015, alone over 51 million vehicles were the subject of safety recalls, more than in any previous year.

By analyzing NHTSA and proprietary J.D. Power benchmarking data using its SafetyIQ platform, J.D. Power has identified these primary factors impacting completion rates for those recalls: vehicle age, vehicle type, overall population of recall, and type of recall.

"The steady surge in recalls, combined with NHTSA’s stated goal of 100% recall completion rates, have made the number of un-remedied recalls still on the road a critical statistic for automakers and dealers," said Renee Stephens, vice president of U.S. automotive at J.D. Power. "By understanding the behavioral trends of vehicle owners, as well as recall completion rates among different vehicle and recall types, as an industry we can better tailor communications to improve those completion rates."

Here are some key findings in the J.D. Power SafetyIQ analysis (all data is based on recall decisions made from 2013-2015, as reported through six quarters of completion information):

• Un-Remedied Vehicles More Common in Older Models: The total recall completion rate for vehicles with model years between 2013 and 2017 is 73%. This compared with a completion rate of just 44% for vehicles manufactured between 2003 and 2007.

• Vehicle Type Plays a Big Role in Recall Completion: Among vehicle segments, large/work vans have the highest overall recall completion rate at 86%, followed closely by compact premium SUVs at 85%. This is a contrast with the mid-premium sports car segment, which has a completion rate of just 31%, and large SUVs, which have a completion rate of 33%.

• Larger Populations Present Bigger Completion Challenges: The completion rate for individual recalls affecting more than 1 million vehicles is 49%. This compares with a 67% completion rate for individual recalls affecting less than 10,000 vehicles. It is sometimes difficult to obtain parts to launch large campaigns. In addition, customers can more easily receive a targeted communication method, such as a phone call, with a smaller population of vehicles.

• Powertrain and Electrical System Recalls Most Likely to Get Fixed: Of the major safety components, the groups with the highest recall completion rates are powertrain (71%), electrical (62%), and hydraulic brakes (66%). Air bags and suspension issues have the lowest completion rates at 47% and 48%, respectively.

"By better understanding the specific factors driving recall compliance among vehicle owners, manufacturers, and dealers can better tailor their communications and manage the recall process much more efficiently," said Stephens. "This is a critical level of intelligence for the industry, which we believe will ultimately help reduce the number of un-remedied vehicles still on the road."