Depending on where you live, traffic lights work by using metal detectors underground to see if there are cars waiting to go through the intersection. This works fine for cars and motorcycles, because they have very large frames. MBs are quite small and often do not trip the traffic lights. In other words, you may find yourself sitting at a red light for a very long time and the light might only turn green when a car shows up.
This motorbicycling.com thread talks about solving this problem. The solution I went with was to place on my bike a large neodymium magnet. These magnets are extremely strong and will sufficiently trip the sensors at traffic lights. All you have to do is obtain one and stick it somewhere on your bike(it stuck under my kickstand just fine). You can get a magnet from gaussboys.com, or you can harvest one from a bricked hard disc drive.
EDIT: Here's a Youtube video by Kip Kay explaining how this is done.
NOTE: Beware when handling strong magnets. Never put 2 neodymium magnets together and beware of getting your fingers pinched when putting a magnet near something else magnetic.
A warning to those looking to mod their motorized bicycles:
Most, if not all, of the hacks and mods featured on this site were made by individual hobbyists. If you are new to working with 2-stroke bicycle engines, please by aware that there's a possibility that a mod featured on this site could seriously damage your engine. Please don't try any mod from this or any other site unless you fully understand what you are doing. The owner of this blog will not be held responsible for both material and bodily damage caused by performing a modification featured on this blog. Also remember that opening up your engine may void your warranty!