According to a new report by Subsidy Scope from the Pew Charitable Trusts, user fees for the construction and maintenance of US highways were at an all time low in 2007. In other words, fees from non-users are at an all time high ($70 billion). In 2007, 51% of funds for construction and maintenance was generated from user fees, down from 71 percent 40 years ago. User fees include the gas tax, while non-user fees are things like bonds from local governments and sales and property taxes.
The main causes of this change according to Subsidy Scope? No increase in the federal gas tax since 1993 and an increasing reliance on state and local governments to pay for roads. People also drive less as fuel prices increase. No mention of increasing use of alternative fuel vehicles, but surely that will play an increasing role in revenue declines in the future.
In contrast, I think 25% of transit’s costs are paid through user fees, although please correct me if I’m wrong. Here is some data to wade through if you’re interested. It should also be noted that both of these percentages include only internalized costs of roads and transit, not externalized (non-monetized) societal costs (pollution, congestion, etc).