A single cloverleaf on the interstate highway today can cost almost as much as the C&O Canal. Are these constructions a good use of public funds?
First of all, please realize that the $11 million cost of the canal in the mid-1800s may be nominally equal to the cost of a cloverleaf, but would actually be much higher if we adjusted for inflation.
But to the main issue here, which is whether such expenditures are worth it. Government spending on infrastructure can certainly be a good use of public funds if it is done correctly. Infrastructure is economically beneficial when it allows commercial activity to occur more easily. This was certainly the case with canals in the early US because they cut transport times and costs by huge margins. This allowed much more economic activity to occur because it made it much more possible to move things (coal, in the case of the C&O) from where they were produced to where they were needed.
In today's world, freeways are vital since so much of our commerce involves goods that are moved by truck. If the cloverleaf in question serves an economically important place, then it is worth the money. Infrastructure spending only becomes wasteful when it is spent on projects that serve no useful economic purpose like the proverbial "bridge to nowhere."