Two issues need to be addressed:
1) moving through lights at a 'safe' speed
2) being in a gear which allows good control of speed with the accelerator pedal
Consider the scenario in which you are on a dual carriageway, travelling at 70 mph, approaching traffic lights a quarter of a mile ahead. They have just changed to GREEN. No traffic is on the carriageway ahead. You intend continuing straight ahead.
MIRROR CHECK - a vehicle is 200 yards behind, with several more vehicles behind that. A reduction in speed is ESSENTIAL, since driving through at 70 mph would be more dangerous than any consequences of slowing. At 70 mph, you would have little chance of dealing effectively with an emergency situation at the junction e.g. other vehicles or pedestrians crossing your path. Also, the driver behind would have little time to respond to your reaction.
These hazards will be present no matter what speed you drive through the lights.
SLOW DOWN by deceleration but WATCH the driver behind. Try not to allow yourself to be pressured to maintain traffic flow when to do so would put you into a dangerous (or illegal) situation. If safe, a reasonable maximum speed would be around 50-55mph. Whatever the speed, the gear in use has to 1) provide good engine braking, in case the lights change to RED or for an emergency situation and 2) provide a reserve of power so that you have a choice of either slightly increasing or decreasing speed in response to a hazard at the junction. The important consideration is CHOICE. Dropping from 4th to 3rd gear would be reasonable for traffic lights generally, but for a dual carriageway scenario at high speed, 3rd or 4th gear would be reasonable (depending on the performance of the vehicle).
By Mark Farnworth
Disclaimer: Driving is never a black and white activity, but full of grey areas, therefore neither I nor my fellow Observers in the St Helens & District Group of Advanced Motorists are liable for any consequences you may experience as a result of reading our advice. You are the driver. You should be in control of your vehicle at all times.