Modern Vespa

The easiest way to trigger sensors is to adjust the position of your scooter on the sensors. You'll have to check out the cutouts in the road and try to determine which type of sensor you're dealing with.

The most common are Dipole and Quadropole Loops. These will usually look like circles or rectangles. A Quadropole Loop will usually be bisected by another cut or look like a side-by-side pair of cuts.

Dipole Loops:
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Quadropole Loop:
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Best positions:
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To maximize the likelihood that an inductive loop sensor will detect your scooter, it is important to position yourself over the most sensitive portion of the loop. There are several common shapes of inductive loop sensors, each with a different "sweet spot" for bicycles and powered two wheelers as shown in Figure 1. The two most common shapes are the dipole loop (Figure 1(a)) and the quadrupole loop (Figure 1(b)). For either of these two loop patterns, position both wheels directly over the sawcut for the wire, choosing either side for the dipole loop and using the center sawcut for the quadrupole loop. (The center sawcut of the quadrupole has twice as many wires in it as the outer sawcuts and is a more sensitive location.) If the signal does not detect you, you may wish to try leaning the bike over toward the center of the dipole loop, or to either side for the quadrupole loop. Newer traffic signal installations usually feature quadrupole loops, which are better at detecting bicycles than the older dipole loops. The third type of sensor loop, the diagonal quadrupole (Figure 1(c)), is designed to provide better detection of small vehicles such as scooters positioned anywhere over the sensor.
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In addition to positioning, you may have some success by placing your scooter in the recommended position then tapping your center stand on the asphalt. This basically moves a mass of metal closer to the inductive loop, which may be enough to trigger the meter.

If you're unable to trigger a light and there's a car waiting behind you, sometimes the best thing to do is scoot up and let the car trigger the light. Watch out for pedestrians and (of course) don;t ride out too far into the intersection.

The following states have laws which permit 2-wheelers to run a light if they're not being detected and if passage is safe (no cross traffic):
North Carolina
Wisconsin
Idaho
Arkansas
Tennessee
Minnesota
Missouri
Virgina
Oklahoma
Illinois (as of Jan. 2012)

The circumstances in which you can run the light varies from state to state, so please consult each states vehicle codes. In some, you have to wait for a certain number of "full cycles"; others have time limits, etc.

Because CA has to do everything differently, the state has a law (California AB 1581, signed into law in 2008) requiring cities to make their triggers sensitive enough to detect all vehicles. Each city has their own means for reporting lights that won't trigger, and the city is required to send someone out to to make the adjustment. (This program is state funded by AB1581.)

Place your scooter on the center stand. Hop off. Run over to the pedestrian crosswalk button. Hit it. Run back to scooter.

Unsurprisingly, there's a lot of debate over whether crosswalk buttons work. We'll set that aside for our purposes.

There are a number of magnets on the market which claim to assist in triggering lights. These are usually neodynium or "rare earth" magnets that can be purchased for as much as $40. The problem with the magnet debate is that these may work, but not because they're magnets.

If you'll recall, induction loops detect metal, not other electromagnetic fields. The traffic light magnets are big, often heavy masses of metal that you affix to the bottom of your scooter. If they work at triggering lights, it has nothing to do with the fact that they're magnetized. Any similarly conductive mass of metal would have the same result.

There are a number of patent applications for devices to trigger inductive loops (links below). None of these use magnets as primary trigger. (Some use magnets or electromagnetism as part of a larger apparatus.)

So do magnets help trip lights? Not really.
Will placing a large mass of metal under your scooter help? Maybe.

It may be best to try some of the other solutions before going out and buying something that may or may not work. (Caveat: All this may differ with electromagnetic sensors, but those are rare in the US and have largely been replaced by inductive loops.)