The Highway Trust Fund was established in 1956 alongside President Eisenhower's monumental interstate project, and has since been funded largely by gas taxes. But due to a combination of changed driving habits and increased fuel efficiency, that trust fund has been yielding smaller and smaller revenues in recent years. "If Congress fails to fund it, it runs out of money," President Obama said Tuesday. "That could put nearly 700,000 jobs at risk."

Without the trust fund, Congress loses a powerful means to keep states in compliance with national standards. Here's a look at some of the issues that have hinged on the fate of the fund.

Drinking Age

Most famously, the Highway Trust Fund was used in 1984 to get states to comply with the new national drinking age of 21. States that did not comply with the Reagan administration's drinking-age law would see 10 percent of their federal highway funds — in some states, several million dollars — cut. All of the states eventually complied, and the U.S. continues to have the highest drinking age in the world.

Speed Limits

In 1974, in the midst of the Arab oil embargo, President Nixon and Congress set the national speed limit at a sauntering 55 miles per hour, in order to ease the demand for gasoline, and tied states' compliance to highway funding. Consequently, according to a paper in the American Journal of Public Health, "road fatalities declined 16.4%, from 54,052 in 1973 to 45,196 in 1974."

In 1995, highway funding was used as leverage again — only this time to states' advantage. The Republican Congress, championing states' rights, authorized funding for the nation's highways, but only if speed limits could be decided by the states. President Clinton signed the measure repealing the national speed limit, but cautioned: "I am deeply disturbed by the repeal of both the national maximum speed limit law and the law encouraging states to enact motorcycle helmet use laws."

Not wasting any time, Montana decided to drop numbers from their speed-limit signs altogether. On rural roads, the state gave the tantalizingly vague instruction that cars should travel at a "reasonable and prudent" speed during daylight hours, thus earning the nickname the "Montanabahn."

One blog post comparing the Montanabahn to the German Autobahn explained the difference between the two superhighways like this: "While you have fewer people to worry about per mile in Montana, you have more animals to worry about per mile, especially at night."

Consequently, traffic deaths in Montana spiked — 1997 was the most deadly year for Montana roads in a decade. In December 1998, the Montana Supreme Court ruled the "reasonable and prudent" speed limit unconstitutional after a man successfully challenged a speeding ticket up to the state's top court, anchoring his case on the vagueness of the law.