Number of lumens is entirely dependent on your surroundings. In urban areas with sodium lights--you need more lumens to see/be-seen. Country or non-lit roads you can run a lower brightness mode. In rain you need more to simply see the surface of the road. This is the upside of multi-mode headlights, you can run them at lower output when you want, and get longer run time.
Remember...car light throwers are in the range of 1K to 4K lumens per side. Your piddly bike light is lower output than them, almost always.