Check your local laws, Though it may be legal ...BARELY anyone I KNOW UNDER THE AGE OF 30 ...even has a clue to what hand signals mean, the kids today would say ..LOOK AT THAT DING DONG IN FRONT OF US WAVIN HIS ARM AROUND!...right up to where they run up the backside of your truck.
Installing working turn signals isnt that hard to do and with todays drivers I suggest ya have em ...but it is your choice.
Heck I went the xtra mile an installed turn signals in my bed rolls with brake lights also ...to keep the not so BRIGHT folks behind me in check.
Just my 2 pennies.
Tim