Roadway user fees imposed on motorists don't cover the costs of roads and the balance is covered by general tax funds. Because of the type of roads cyclists use and the funding that pays for them; as well as the low costs imposed by cyclists and pedestrians on the road network and society, it is actually they who subsidize motorists. And the roads they get are built to the needs of the subsidized.
There are several analyses that show that "user fees" - such as the gas tax, tolls and vehicle taxes and fees - cover only about 50-60% of road costs. [The Brookings Institute, 2003, 58.9%; Subsidy Scope, 2007, 51%; The FHWA, 1999, between 72% and 22%]. The remainder comes from other taxes and fees, property taxes and borrowing. So non-motorists do pay into the building of roads. In fact, a study by the Victoria Transport Policy Institute showed that motorists are heavily subsidized by non-motorists.
And, cyclists do pay taxes. They pay sales taxes on their bikes, bike supplies and even the calories they burn while cycling. Since there is often no sales tax on gasoline, the taxes cyclists pay per mile might in fact be very close to what drivers pay per mile. Bicyle sales alone is a $6B industry, meaning that cyclists have paid about $300M in sales taxes on their bicycles. This money is added to general tax revenue and general tax revenue is used....to build roads.
To add insult to injury, the roads that cyclists subsidize are not built for cyclists needs. On the one hand, they're overbuilt - wider and capable of carrying greater weight than cyclists need; and on the other hand they're not built or maintained to a standard that considers cyclist safety - with tire catching storm grates added and cyclist-throwing potholes allowed or temporary steel plates installed, for example.
"Victoria Transport Policy Institute" is Canadian. Things mentioned may be different in the US.
"Since there is often no sales tax on gasoline,..." ??? There sure is around here.