Doing so allows the government to raise more money for highways by allowing employers a range of return rates to estimate their pension liabilities. Those that assume higher returns are able to reduce the amount of money they pay into their defined benefit plans. That, in turn, limits the amount of pension funding sponsors write off, raising their corporate taxes. Higher taxes help pay for highway funding.

The lower pension funding contributions that result are expected to generate more than $6 billion in tax revenue, according to a Joint Committee on Taxation estimate. Plan sponsors will now be able to take advantage of the law through 2017.

Also read: MAP-21: The wrong course for pension plans?  

The House version of the bill had met unexpected resistance over pension-smoothing, which is often decried as budget gimmickry by members of both parties. 

Marjorie Martin, an actuary with Buck Consultants, said that while some plan sponsors may be happy to see their immediate pension liabilities reduced, ultimately, sponsors are going to have to make up for the reduction in funding. 

"What they don't put in now they're going to have to put in later," Martin said Friday, adding: 

"If future investment returns are not sufficient to make up for lower contributions today, sponsors' contributions in the future will have to be larger." 

Had the Senate not capitulated to the House version of the bill, states were expected to see immediate cuts in money from Washington to pay for transportation projects. Bloomberg reported that, while the legislation ensures federal funds continue to flow to states for a few more months, it delays the more contentious debate over long-term financing for infrastructure needs.

Companies including United Parcel Service Inc. and groups like the National Association of Manufacturers and the AFL-CIO have been pushing to lock in funding for as many as six years. President Obama is expected to sign the legislation into law shortly.