The Congressional Budget Office (CBO) predicts that the Highway Trust Fund will be empty by 2014. Federal gas taxes support the fund and are greatly outpaced by spending. These predictions of the trust fund demise are getting to be a regular thing.
Link to the CBO report
Additional posts and articles
Boxer Files Highway Trust Fund Extension
CBO reports highway trust fund headed for bankruptcy in 2014
Highway Fund in Continued Trouble; Empty by August
Returning to Dust: Our Aging Infrastructure