I think you need something like 200 lumens, give or take. That gets you in the same order of magnitude as a car head light. Then you have to decide how to power it. Options include throw-away batteries (probably not the best option), rechargeable (with separate battery pack or in a self-contained package) and dynamo powered (a charging system that stays with the bike).

I have two lights I ride with at night; one is a Bush & Mueller dynamo-powered light (on my commuter bike). This is a fairly expensive and inflexible option since I have a SON dynohub built into my front wheel of that bike.

The other is a CygoLite Rover II which I can switch out between my other bikes. It has a battery back that sits in a water bottle cage (which I'm not crazy about) and a light unit that clamps onto the handlebars. There were versions of the Rover that have battery packs that simply velcro to the frame which would be my preference.

I think some of the more recent offerings are self-contained light and battery units that can be charged off a USB port. I would look into that if I were in the market for a new light.