Too much carbon-fibre in your bike and not enough ferrous metal to trip those in-road traffic light sensors? Well woowoowoo just pointed me in the direction of "Traffic Light Trigger for your Bike" over on instructables. Basically the secret is to glue a rare-earth magnet to the heel of your shoe.
And as an added bonus, it'll probably cure your arthritis, restore your thining hair, and make you last longer in bed too!