Auto Not Engaging Reverse!

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We I nearly had my nose swiped off a few days ago facing a slight slope down from a street onto the main road I thought I reverse back a bit,put it in reverse and to my amazement it went forward instead FMD! I then decided to do it at home as the driveway had a similar slope and yep it did the same thing its needs a few feet to engaged reverse next time I will apply the handbrake first. Anyone here know why this happens with autos!!

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ps I had to stick my nose out to see cars on my RHS coming off a side road onto the main road I'm trying to get on