Mass transit combined with congestion charges make the most pure economic sense.
Highway space is a limited and expensive good. Building more for the limited period during the day when people are using full (or more than full) capacity is a very poor economic choice. Most highway capacity is unused for most of the time.
Therefore encouraging people to use unused capacity or discouraging use when capacity is full via higher costs is a legitimate and economically conservative way to solve the problem. Mass transit also makes economic sense as a good use of public money as an alternative to those who cannot afford high congestion feed.
More highways mean higher long-term taxes due to maintenance costs. The alternatives pay for themselves.
You cannot be a fiscal conservative and favor more highways that are empty most of the time (for those of you who wish to contradict, please be aware there are 24 hours in a day).
I've long suggested that highways should be built the same way railroads were laid across the country -- with FREIGHT MOVEMENT considerations being the primary factor in decision-making, and PASSENGER MOVEMENT being accommodated as a secondary factor.
The same could be said of mass transit also.
The carpool lanes were a good idea on paper, but the fact is, most do not want to give up their independence, and the lanes lay there unused, while traffic crawls just one lane over.
You obviously haven't travelled on the same highways I use. I don't believe anyone is talking about "empty" highways. They are talking about the ability to travel on a freeway (65 mph speed limit) at a speed greater than one is required to travel in a school zone (25 mph). It is currently not uncommon to travel for two hours on that freeway, in stop and go traffic, only to make it 50 miles.
BTW, there is hardly such a thing anymore as "rush hour" in some of these areas. You can experience the same thing most any time of day.