Currently the vast majority of the lights are of the standard incandescent type (pictured above), but are slowly being replaced with longer lasting and more energy efficient LED bulbs (pictured below).
The standard incandescent bulb used in traffic signals currently uses 50 Watts of power and lasts just 12-18 months. LED light fittings however are easier to see in daylight and poor weather conditions, use just 17 Watts of power, and last well over 5 years.
The costs for a standard red-amber-green head is currently around £150 compared to over £600 for an equivalent LED model however LED prices are falling rapidly. Therefore, although the initial up-front costs are more for LEDs, overall costs are lower thanks to a reduction in energy used and far lower maintenance costs.