18m is just not enough. In-game it feels like a pinky-length, and at 18m it might as well just be disabled. In past Halo games 25m has been the standard, and it was for good reason.
I know that the range was greatly reduced after the beta because of the (ridiculous) 35m range, but 343 you over-corrected. You are bouncing between two extremes, and you need to balance back out at the tried-and-true 25m.
At 18m you either already know someone is there or it is too late. If somone is moving around in front of you within 18m, then you already see them or hear them. If someone is within 18m behind you, then you are already dead; there is nothing you can do (with certainty) to prevent your death. They are already shooting you well before 18m, and you can’t do anything quick enough to stop a sprinting SPARTAN from charging your back for an instant-kill at that range. So now instead of being able to skillfully engage a player approaching from behind, I have to just try to flee, which, when combined with the small map sizes and the fact that your shields don’t recharge if you sprint, simply results in either being chased down and murdered by the initial assailant or walking face-first into another enemy. Such game-play is not competitive or skill-based; it is simply who-sees-who first, which is highly frustrating and heavily determined by luck.
In other Halo games, I was able to remain level-headed and move around with confidence because I knew that I could engage enemies in front of me at any distance, and if someone came from behind I would at least have fair warning and be able to put up a fight. With an 18m motion tracker range, I am now frantically looking around in all directions, unable to get any peace-of-mind from the information provided by my motion tracker, and nine times out of ten I am engaged from the position that I was just looking but had since turned away. It is really frustrating because they are also nine times out of ten within relatively close proximity, but a measly 18m range just isn’t enough to see them coming.
I can honestly see where you were coming from with the 35m range, because players now move faster than ever, so in order to provide a fair warning the tracker should need a greater tracker range. This simply cannot be done though, because the motion tracker has to be balance for map size. You obviously already know that because you reduced it, but 18m is just too small. I’ve been playing Halo multiplayer since Halo 3, and I have etched in my mind a general consensus of what my 25m motion tracker tells me; it is as strong as any muscle-memory habit and twice as hard as any other to unlearn. It is so frustrating to look at my motion tracker and think to myself “all-clear” and then instantly be shot by a player from just around the corner; in my mind there was no physical way that a player could have been sprinting by there. I have grown to trust my motion tracker, and when it suddenly starts feeding me (for all my personal intents an purposes) misinformation, I get really annoyed. My motion tracker is like a sixth-sense in Halo, and for it to suddenly be re-calibrated is disorienting and frustrating.
Please return to the 25m motion tracker range; it is tried-and-true, and it makes for more competitive and skill-based engagements rather than “I saw you first”-kills.