According to the latest forecast from the Congressional Budget Office, the Highway Trust Fund will have insufficient revenues by the end of 2015, but should be able to fulfill its obligations through 2014.
At the beginning of this year, the HTF held $10 billion and the CBO forecast expects that amount to dwindle by half by the beginning of 2014. The CBO expects those funds to have dried up at some point in 2015.
You can view the full PDF of the forecast by clicking here.