Slugging, also known as casual carpooling, is the practice of forming ad hoc, informal carpools for purposes of commuting, essentially a variation of ride-share commuting and hitchhiking. A driver picks up these non-paying passengers (known as 'slugs' or 'sluggers') at key locations, as having these additional passengers means that the driver can qualify to use an HOV lane or enjoy toll reduction. While the practice is most common and most publicized in the congested Washington, D.C. metropolitan area, slugging also occurs in San Francisco, Houston, and other cities. Slugging, also known as casual carpooling, is the practice of forming ad hoc, informal carpools for purposes of commuting, essentially a variation of ride-share commuting and hitchhiking. A driver picks up these non-paying passengers (known as 'slugs' or 'sluggers') at key locations, as having these additional passengers means that the driver can qualify to use an HOV lane or enjoy toll reduction. While the practice is most common and most publicized in the congested Washington, D.C. metropolitan area, slugging also occurs in San Francisco, Houston, and other cities. In order to relieve traffic volume during the morning and evening rush hours, high-occupancy vehicle (HOV) lanes that require more than one person per automobile were introduced in many major American cities to encourage carpooling and greater use of public transport, first appearing in the Washington D.C. metropolitan area in 1975. The failure of the new lanes to relieve congestion, and frustration over failures of public-transport systems and high fuel prices, led to the creation in the 1970s of 'slugging', a form of hitchhiking between strangers that is beneficial to both parties, as drivers and passengers are able to use the HOV lane for a quicker trip. While passengers are able to travel for free, or cheaper than via other modes of travel, and HOV drivers sometimes pay no tolls, 'slugs are, above all, motivated by time saved, not money pocketed'. Concern for the environment is not their primary motivation; Virginia drivers of hybrid automobiles are, for example, eligible to use HOV lanes with no passengers. In the Washington area—with the second-busiest traffic during rush hour in the United States and Canada as of 2010—slugging occurs on Interstates 95, 66 and 395 between Washington and northern Virginia. As of 2006, there were about 6,459 daily slugging participants there. In the San Francisco Bay Area, with the third-busiest rush hour, casual carpooling occurs on Interstate 80 between the East Bay and San Francisco. As of 1998, 8,000 to 9,000 people slugged in San Francisco daily. However, after bridge tolls were levied on carpool vehicles in 2010, casual carpooling saw a significant decline and etiquette became more uncertain. Slugging also occurs in tenth-busiest Houston, at a rate of 900 daily in 2007, and in Pittsburgh. Slugging is shown to be effective in reducing vehicle travel distance as a form of ridesharing. Slugging is more used during morning commutes than evening commutes. The most common mode that slugging replaces is transit bus. David D. Friedman's The Machinery of Freedom proposed a similar system (which he referred to as 'jitney transit') in the 1970s. However, his plan assumed that passengers would be expected to pay for their transit, and that security measures such as electronic identification cards (recording the identity of both driver and passenger in a database readily available to police, in the event one or both parties disappeared) would be needed in order for people to feel safe. Although slugging is informal, ad hoc, and free, in 30 years no violence or crime was reported from Washington D.C. slugging until October 2010, when former Sergeant Major of the Army Gene McKinney struck one of his passengers with his car after they threatened to report his reckless driving to the police. The term slug (used as both a noun and a verb) came from bus drivers who had to determine if the people waiting at the stop were genuine bus passengers, or people wanting a free lift, in the same way that they look out for fake coins—or 'slugs'—being thrown into the fare-collection box.