travelling or traveling

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.

Examples

U.S

On average, it traveled 4 to 5 miles an hour.

Outside the U.S.

Rousteing is very young, and perhaps not particularly well travelled.