If you take away all of the pork barrel items, the social safety net, corporate welfare, student loans etc. you end up with about 1/3 of the Federal Budget.
So the Federal Income tax has NOTHING to do with highways. It is used to reward supporters of the party currently in power. Democrats favor more social spending because the folks that vote for them work in that business to a greater percentage. Don't delude yourself into thinking they actually give a crap. Reverse for Republicans. Their core is large international corporations & the religious right so social spending decreases, defense work goes up and more laws are written to protect their friends from lawsuits. Don't get swept up in the international corporate conspiracy crap either. It's about rewarding friends and punishing foes at the expense of those of us who actually work for a living.
The secondary point is that if the Federal Government stuck to its original role as defined by the Constitution we wouldn't NEED a Federal Income Tax. So continue to grab your ankles every April 15th and smile and wave to all of the other sheeple.