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. Typically slugging is motivated by an incentive such as a faster HOV lane or a toll reduction. While the practice is most common and most publicized in the congested Washington, D.C. 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 built in many major American cities to encourage carpooling and greater use of public transport; the first were built in the Washington D.C. metropolitan area in 1975. The new lanes, 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 passenger.