Larger speed bumps sound like a good idea; however, please check with your local fire department before increasing the size. Some areas have restrictions due to the damage speed bumps can cause to a fire truck and/or ambulance.

Alternatively you could make it a contest for on-site staff to find the most violators. And then give them a prize. I used to have a boss that would randomly show up on site and tell us whoever finds the most lease violations and gives that apt the appropriate letter (1st, 2nd, 3rd warning/violation), that employee would get a portion of the fine.
Maintenance always won because they were outside more and could see trash bags by front doors/on patios, and all of the grills we didn't allow.

I would put up a bunch of stop signs. Then, when they fail to stop, fine them. It is easier for people to understand that they need to stop than drive 5-10 miles an hour. I also like the big speed bumps.

Have someone paint this.. as soon as they get used to it.. put the real things up or dig a real hole