I know a little bit about dogs, this goes double for Pitbulls.

"foaming at the mouth" isn't a determiner for attack behavior, it is a sign the the dog may be just a little over heated or excited. It is caused by drooling when slightly dehydrated, and bull breeds don't need an excuse to drool. It would also be hard to see from the distance you speak of. Unless it was copious, and then it would be a sign of rabies, which I can just about promise you wasn't the case.

I don't doubt that the dog was snarling or barking at you though. Did the dog leave its property to chase you, or was it just keeping watch over where it lived? If it left its property to chase you, then the owners are at fault, and you should absolutely alert your SPCA that there is a roaming dog, Pitbull or otherwise.

Let's not let this thread unravel into an anti-Pitbull thread though. This event could have happened with any breed of dog. Any breed can be raised wrong. Any breed can be breed wrong. Any large athletic breed can be a danger when this happens.

I think the most important thing to answer is if this dog left its property or not. If it did, there is a lot you can do to protect yourself, and the dog, from the dog's neglectful owners. Even a well intentioned and non-aggressive large dog can pose a threat to itself by playfully chasing down a stranger and getting hurt or killed by that stranger in the ensuing lack of communication. I'm not saying by any means that is what happened to you, but it does happen as a result of owner neglect sometimes. If it didn't leave its property, yet let out a scary warning, it sounds like the dog was well trained and behaved enough not to be an actual threat.

In general, and there are exceptions to every rule, I find the following to be most helpful to me in preventing a dog attack while cycling. But I am an alpha dog, so YMMV.
A) Try to calmly move away from the property line the dog may be protecting. And by no means step onto the property.
B) Calmly place the bicycle between you and the dog if the dog approaches, otherwise stay on the bike .
C) Look the dog directly into the eyes, if it has still not backed down and is intent on engaging you. CALMLY, yet firmly, say "No" and/or "Bad dog". Do this without fear and dominantly for best effect. Try not to raise your voice too high, as you may come off as too shrill or excited. The point is to show the dog that you are the alpha and must be listened to. If you believe that you are, or can fool the dog into thinking you believe you are, this may be all that you need to do.
D) Offer a treat to the dog if it has calmed down or isn't snapping at you. Offer some pepper spray to the dog if it is snapping at you. If you know how to use hackling or the difference between growling and play barking as a determiner, use this as a litmus test. If dog behavior isn't your forte, err on the side of caution. I also find a game of "fetch" with your pump or a power bar can fool a dog long enough for you to get out of dodge. This is especially true of terrier breeds (a breed group that Pitbulls are a member of), as they can be total slaves to stimuli.
E) When you are a safe distance away, call the SPCA right away, so they can catch the owners in the act (or not long after the act) of neglecting to control their dog.
F) If the owners come out to restrain their dog, act very clam and nice towards them, even if they are in the wrong, so as not to give their dog a reason to think it must protect its owner and escalate what now may be a more controlled situation.

Just my $00.02, but it has kept me out of trouble. Then again, I have a bit of experience working with big dominant dogs with high drive. In my avatar, you will see my current dog. His name is Cassius and he's a German Shepherd Dog. He's the one sitting nearest the camera. And yes, they are sitting on an airplane.