The rules of grammar are when you added (ed) to a word,
Plan ------> Planed
Pin -------> Pined
Slop ------> Sloped
and leave the final consonant single the vowel is sounded it's upper form.
However if you double the final consonant it's pronounced in it's lower form.
Plan ------> Planned
Pin -------> Pinned
Slop ------> Slopped
So we apply the rules to,
Travel ------> Travelled The English pronounce the 'e' as a lower 'e' as in 'bell'
Travel -----> traveled The Americans pronouce it as 'ee' as in 'wheel'