Most transportation bills run for up to seven years.  This is so state agencies can do the long term planning necessary and be assured of funding.  The last one SAFETEA-LU ( Safe, Accountable, Flexible, Efficient  Transportation Equity Act- a Legacy for Users)  (honestly, you can't make this stuff up!)before the current one ran for far longer, because Congress couldn't agree on a new bill, so they kept extending that one for another 2-3 years.  Unfortunately, it kept spending at the same rate, without generating any additional revenue (taxes), so it began depleting the Highway Trust Fund pretty fast.
When they finally passed a new transportation bill (MAP-21 aka Moving Ahead for Progress in the 21st ) (another silly title) they only made it for two years, because no one wanted to address the fact that the Highway Trust Fund would be at zero after just a couple years.  It expires this October, and the word is they will just do continuing resolutions on it for another year or two because it is an election year and no one wants to deal with increased taxes or fees.
Of course no one wants to pay more taxes or fees.  However, federal gas taxes have not increased in many years.  Inflation and asphalt and concrete costs have skyrocketed.  And we are driving less and vehicles are more efficient.  This results in less $ paid to the highway trust fund.   The overwhelming $ from the trust fund goes to pavement. But yes, highway trust fund  money also goes to mass transportation and bike trails, etc., 
Is money wasted in other Federal programs that could go from the general fund to the highway trust fund?  Of course.  Congress will have to either gore someone's ox by cutting some favorite program of someone's, or gore all of our oxen by raising gas taxes or fees.  There are literally billions or more likely trillions of dollars of deferred maintenance on our road infrastructure out there and it will either need to be fixed or we will have more bridge collapses like we had in Minneapolis on I-35 a few years ago.
It ain't a pretty situation, and the solution is probably going to make nearly everyone unhappy.