WASHINGTON—The tax-favored status of an employee benefit tapped by those who use mass transit to get to and from work would be boosted under a provision in highway funding legislation approved Wednesday by the U.S. Senate.

Under that provision, included in S. 1813, employees would be able to reduce their taxable salaries by up to $240 a month to pay for mass transit expenses.

The monthly maximum contribution is $125. The new higher limit would expire at the end of 2012.

The higher contribution limit would be retroactive to Jan. 1, 2012, though the legislation isn't clear about how employers would allow retroactive contributions. The measure does not require employers to allow retroactive contributions and many, because of administrative complications, may not, experts say.

“There is no restriction on an employer's ability to fund less than the applicable amount. Thus, we think employers can take the position that they won't fund up to the statutory limit this year. In other words, we think employers aren't required to offer retro pretax and likely will not opt to do so,” said Jennifer Henrikson, assistant general counsel-outsourcing legal at Aon Hewitt in Lincolnshire, Ill.

Congress on two prior occasions boosted the pretax contribution limit for mass transit expenses.

Under a provision in a 2009 economic stimulus law, employees were able to reduce their taxable salaries by up to $230 a month to pay for mass transit expenses. At that time, the maximum contribution was $120 per month.

Then, just prior to its expiration in 2010, lawmakers agreed to continue the higher mass transit tax break through the end of 2011 as part of a broader measure that temporarily reduced payroll taxes. That higher limit expired at the end of last year.

The broader bill, which now goes to the House or Representatives for consideration, also includes a provision that would change the methodology for employers to value their pension liabilities, which, if passed, would significantly reduce the amount of money many employers would have to contribute to their plans during the next few years.