I think the safest thing - and the most polite - is when they first start to signal, make a determination if you are able to safely give them space by easing off the gas and maybe lightly tapping the brakes (of course you don't want to do any hard braking because that creates a risk of being rear-ended). And if the answer is yes, then let them merge. If you're traveling at a safe speed, leaving a safe distance to the car in front of you, and assuming they signal about the time the car in front of you passes them, this should be doable more often than not.