A lot more than 100 lumens!To ride at 25mph, on unlit roads you will need more than 100 lumens.
The problem, until recently, was that decent lumens were expensive. As a result, you could accept using street lighting to see where you are going leaving the high lumen lights for the unlit roads . I propose that with modern led systems, you can afford a powerful front light. The lights will ensure you are seen and less likely to be ignored. Look at the Cree systems on Amazon - for around £20 you can get a decent amount of lumens.What would you recommend in and around traffic. The roads will be well lit
The problem, until recently, was that decent lumens were expensive. As a result, you could accept using street lighting to see where you are going leaving the high lumen lights for the unlit roads . I propose that with modern led systems, you can afford a powerful front light. The lights will ensure you are seen and less likely to be ignored. Look at the Cree systems on Amazon - for around £20 you can get a decent amount of lumens.
Bit of a sweeping statement there; do a search for Cree on Amazon and look at the options. You don't need to be shining at the cars to give a decent amount of light out that will ensure you are noticed.My biggest concern about the cheap ebay lights is that they have terrible optics. They throw light everywhere and they dazzle other road users.