Sorry, HR, but my take is that in this act of bribing, there was harm entailed.
First, the harm to yourself is that ,by paying the bribe money, you have already made it easier for you to do it again the next time and the next time. Hence it will mean increasing your tendency to act irresponsibly and to try and escape from the pain of punishment/fine of a big sum for speeding.Paying less, like in this case, won't act as a strong reminder and deterrent for you not to speed in the future and this might lead to harmful consequences of accidents to yourself and others.
As for the policeman, by paying him a bribe ,you are encouraging him to be corrupted and to go around collecting bribes from other people who will also be causing harm to themselves like above.
Finally, if you had paid the fine, you would not have to worry about committing the non-virtuous action of lying by not reporting the policeman to the authorities.
So my take is, to spare oneself from committing two non-virtues, it's better to pay the fine! Sorry HR. I may be a bit too 'far-fetched' in my argument above.