They can ticket you because it is your responsibility to know what your speed is. Not the responsibility of the officers to inform you. So although you aren't required to have a speedometer, you better get one if you think you are going to exceed the 15mph. I would also guess they arent going to ticket you for 16mph, but for sure they will at 20mph. Basically, they just want to keep the speed on the MUP reasonable.
Is this a bike only law though? Or is it a 15mph limit on anyone on the MUP?
Also, I agree that 15mph is reasonable. The whole reason I dont like to ride in the parks around here is because I dont think it is safe when I am on the trails if it is busy at all. Slow joggers, walkers, and dogs on leashes constantly cross the yellow. I neither want to get hurt, or hurt someone else.
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2008 Cannondale System Six
2016 Pivot Mach 5.5