In American English, the inflected forms of travel take one l—so, traveled, traveling, traveler, etc. In varieties of English from outside the U.S., these forms take two l’s—travelled, travelling, traveller, etc.
According to the ngram below, American English adopted the one-l forms in the early 20th century. Many other verbs ending in -el went through a similar transition around this time. Others, such as cancel, did not change until several decades later.
This ngram graphs the use of traveled and travelled in a large number of American texts published from 1800 to 2000:
On average, it traveled 4 to 5 miles an hour. [Los Angeles Times]
Morgan claimed the French vessel has been towing at 4 nautical miles, whereas the tugs could have traveled at 6 to 7 nautical miles. [Newsday (dead link)]
But perhaps the most logical of all explanations is that Romney is a time traveler. [Washington Post]
Outside the U.S.
Rousteing is very young, and perhaps not particularly well travelled. [Telegraph]
The $60-million project has been under way since May 2010, snarling traffic and stalling commuters travelling the busy corridor. [Globe and Mail]
That makes him ideally placed to answer the questions every harried air traveller would love to ask. [Sydney Morning Herald]