Visual Estimation of Speed from 20-30 Feet at 15-20mph Speeds

[AZCode]

My question involves a speeding ticket from the State of:
Arizona

I just was pulled over in a temporary school zone (they put signs during certain times of the day) for allegedly speeding. He said I was going fast and I said no I dont think I was, till the end of the zone where I started accelerating. He said he watched me going into the zone (total 30-40 feet) and I entered fast. I was crusing with 20-25 and dropped to 15 in the beginning of the zone. He was on the other side of a road (appx 20 feet away from the lane I was in) said I was going with 24-25 probably but he said he'll give me a huge break and give me a regular speeding ticket instead of a double-fine and put 20+ for my speed on the complaint. The limit was 15mph and my question is what are my chances if I fight it at court myself? or with an attorney? how accurate can an officer be at visual estimation of speed at such close distances and slow speeds? Is his judgement of 5+mph in these conditions enough to convict me of speeding?

Thank you all for your advice in advance!!

John

[AZCode]

and it writes "OR" under "Speed Measurement Device" on the complaint. Does anyone know what that stands for?