You could make the argument, but a court will likely determine you had ample time to lower your speed (if you were driving attentively) prior to reaching the speed limit sign.
You can make that argument- but you will lose. You will be admitting that you were speeding. You might fare better fighting the ticket with the help of a traffic ticket attorney.
Do you want to plead guilty or do you want to fight the ticket? You can always go to court and plead guilty. Most traffic judges/commissioners reduce the fines at arraignment anyway.
However, I think you my two colleagues were too quick to dismiss your chances of fighting the ticket. You did not post a lot of facts, but I can think of several ways to fight a ticket like this. While the methods for fighting radar tickets are too numerous to describe in any helpful detail here, you can fight it on the grounds that your speed was reasonable under the conditions (weather, time, vehicle and pedestrian traffic, etc.) or, if there was no valid traffic survey justifying the low speed, you could argue it was a speed trap.
I handle traffic cases in addition to criminal cases. Feel free to contact my office at (415) 565-9600 and ask for Christopher Dove.