A car headlight is 1300 lumens (who cares about the watts). You wouldn't want to drive your car with only one headlight working half way (i.e. 650 lumens) and go 30mph or faster, would you?

1500 lumens is a reasonable light standard especially if you ride on the road. Most of the higher lumen lights have a wide spread on the light so you really need to think more in terms of light density - i.e. lumens per square area. I have a flashlight with at about 200 lumens with massive throw and pencil beam (narrow). It's lumens per square area is higher than my Lupin Wilma at 1500 lumens which has a very wide beam similar to a car headlight. The narrow beam is useless for riding (although great for chasing vandals off my dock) while the Wilma is great for riding but pretty crummy when long throw with high lumens is needed.

Really, if you are riding on the road, 1500 lumens should be reasonable. 2600 lumens should also be fine, about what a car with two headlights produces provided that they spread the light out - like a headlight should. If you ride on a trail where it's confined (i.e. riding in the green tunnel) then you can definitely get by with less because the light can't spread out. But you still probably need the same lumens per square area on the trail it's just that your area is smaller so you require less lumens.

I do a lot of road riding and I'm wanting about 3000 lumens or so. I'm going to aim them appropriately and I'm not going to apologize to anyone that complains if they are not complaining about standard car headlights.

J.