I was taught that if you are going to do this, it's your responsibility to get up to speed or faster quickly. It's a courtesy thing.Hate it when drivers pull out in front of me while I'm on the main road and going at a reasonable speed. If I have to slow down significantly to not hit them, it means they didn't actually have enough time to do that. What's even worse is when they do that and then proceed to drive super slow as a final insult.
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Phoenix, AZ drivers take this to extremes though. They pull out in very short amounts of time then redline it to get going faster than you are.