It depends on the shape of the loop. Most of the loops in my area are figure 8 (three lines in the pavement) or square (two lines in the pavement). On figure 8, roll the wheel directly over the center leg for as long as possible. For square, I roll the wheel over either leg for as long as possible. There are only a couple of places in Denver that use the round loop but I've never found them to be any less sensitive. If anything they seem to be easier to trip by simply riding over the loop at any point.

I've used this method in 47 of the 50 states and never had a problem tripping a light if I could see the loop as I clearly stated above. Even repaving doesn't necessarily mean that you can't find the loop, however. On new fresh pavement, it is impossible. But if the pavement is a couple of years old, often times cracks will develop over the loop lines. Often I can see long linear cracks in the pavement and I know that is where the loop is. My success rate with those is a little lower but not by much.