You need to get a meter and measure the voltage of the battery and also when engine is running, or it will just be guesswork. I have a DVR2 and without lights, it gets up to 14.6 volts at around 2000+ RPM.
Don't forget that running a 55 watt headlamp with a pilot bulb (noted the LED rear lamp) you leave very little output to charge the battery and the dynamo will have problems maintaining the correct voltage to the battery or the headlamp.
Once charged a good 5AH battery should in theory, have sufficient capacity to run a headlamp for an hour, in reality you can expect a good 30 mins, so the stops at the traffic lights should not present a problem.
Batteries as they age or have been abused, can give a good terminal voltage (but normally slightly lower) but will have seriously reduced capacity, that's why the best way to test a battery is with a load tester.