Answer:
A costs and benefits analysis is quite simple, if I am going to invest $1,000 in doing project X, what benefits will project X yield, i.e. how will I be better off by carrying out project X.
In case of a highway, they are extremely expensive to build and to maintain. If a private company would build and maintain them, they would need to charge a high amount of money to everyone that uses them. When we are driving on certain highways we pay money at toll booths, but it the highway was private, the price would not be just a couple of dollars, it would be much higher.
E.g., how many people would use a highway if they had to pay $10 for every mile that they drive? Probably very few.
That is why highways are built, financed and maintained by the government. They are extremely expensive to build and maintain, and extremely cheap to use, which is a deadly combination for any for-profit business.
If the government didn't build highways, then we would either be forced to pay very high prices for using them or no one would build them at all.
