Aided by the growth of high-deductible health plans, group health care plan cost increases are easing.

In 2012, group health care plan costs increased 4.1% to an average of $10,558 per employee compared with $10,146 in 2011, according to a Mercer L.L.C. survey of more than 2,800 employers released earlier this month.

This year's average cost increase is the smallest in 15 years, the New York-based consultant said, and is sharply less than 2011, 2010 and 2009, when costs rose an average of 6.1%, 6.9% and 5.5%, respectively.

Benefit experts attribute the slowing of increases in group health care plan costs in part to the growth of consumer-driven health plans. This year, 16% of employees were enrolled in CDHPs, up from 13% in 2011 and more than double the enrollment just four years ago, when only 8% of employees were covered in the plans.

With CDHPs, in which a health savings account or a health reimbursement arrangement is linked to a high-deductible plan, employers can reap significant savings when only a small number of employees switch to CDHPs, Mercer noted. That is because CDHPs cost about 20% less than traditional plans, such as preferred provider organization coverage.

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A double cost saving benefit can be achieved through CDHPs, benefit experts say. The first is that the high deductible brings down the total cost of the plan, while also making employees better consumers of health care services by exposing them to a greater share of the costs.

“When employees have more accountability — more skin in the game — they become more careful consumers,” said Michael Thompson, a principal with PricewaterhouseCoopers L.L.P. in New York.

Other factors also are helping hold down costs. For example, the growth of retail clinics allows employees to get needed medical care during times when their doctors' offices are closed while avoiding trips to hospital emergency rooms, where costs would be many times higher than the clinics, said Helen Darling, president of the National Business Group on Health in Washington.

Yet another positive factor in the health care cost control fight has been the redesign of health care plans in which employers require employees to use what are known as centers of excellence for certain high-cost procedures for employees to receive full or greater coverage. That results in better outcomes and, ultimately, lower costs, said Tim Nimmer, chief actuary for Aon Hewitt in Denver.

Others note that the sluggish economy and continued relatively high unemployment also may have helped to slow health plan cost increases.

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“Utilization of services has slowed down as household income has gone down,” Ms. Darling said.

Other survey findings include:

• Health care plan cost increases vary considerably by region (see chart). For example, costs in 2012 surged by an average of 7.7% for large employers in the West, the highest percentage increase of any region. On the other hand, health care plan costs for large employers in the Northeast increased by just 1.5% — the lowest of any region.

One possible reason for that sharp difference is that CDHP enrollment among large employers in the Northeast jumped more than 50% in 2012, rising from 11% to 17%, while CDHP enrollment among employers in the West remained flat at 10% of employees in both 2012 and 2011, said Beth Umland, Mercer's director of research for health and benefits in New York.

• Fifty-six percent of employers said they would consider providing coverage through private health insurance exchanges. The appeal of private exchanges, Mercer said, is that they allow employers to offer their employees a broader choice of benefits while allowing health insurers to compete for their business.