I think that etiquette wise it's OK. For the most part you're not going to be right next to the same runner for very long so even if it bothers them they won't be putting up with it for any length of time. I have seen other runners use them. Having said that, personally it would drive me bonkers if someone was running next to me and their high HR alert kept going off. If it was a single beep intermittently to given you distance alerts or time alerts, no problem.
OakLeaf, you're doing a 5K and this is your 1st one, right? What are you planning on using the audible alerts for? What's the farthest you've run at one time? How long have you been training for this? Do you have other races planned in the near future?
I ask all these questions because, if you have a decent base and know that you can complete the 5k no problem, your legs are tough from running for a longer time and you don't have another race planned soon... I would forget about HR and just push at 90% effort up until the last 1k and then kick it inot 100%. But my legs can take it from years of running and I'd still recover quickly. If you don't have much base you could injure yourself doing this and be out of commision for a while.
Be aware that if you're planning to use HR to pace yourself on race day, and you're excited, your HR will not reflect your level of effort the same as when training. It will be higher than normal. So a lot of times level of effort is a better way to pace on race day but this is very difficult for newbies.
Good luck and feel free to ask more questions.
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