I've video recorded my lights at sunset facing into the glare of the setting sun, and at night while parked and riding up to 100 yards away, and observed them without a camera. Based on those observations, and reactions from cars, I use whatever the rapid flashing mode is for the light. It's the most noticeable and least likely to induce perspective or proximity error.
Steady lights are too difficult to quickly estimate distance. Particularly single lights. Placing lights around 6" or so apart - horizontally or vertically - may help lend some perspective for drivers if you prefer steady lights.
Regarding flashing interval/frequency, I'd guess that Bontrager, Blackburn and others have the right idea with irregular rapid flashing modes. In my videos I found it easier to spot and estimate distance when the Blackburn 2'Fer was used in rapid flashing mode, along with a couple other red LEDs that have only regular patterned slow and fast flashing modes. The combination of lights placed 6" or more apart, along with the varying flashing patterns, was very visible out to 100 yards in my test videos, at dusk and night.
In daylight I've been using the front facing white lights in flashing mode. Several times I've noticed drivers pause before leaping into traffic from parking lots or intersections, and so far I've experienced none of the close calls I used to have when riding without lights during the day. However at night I'm more inclined to use steady front facing white lights unless I'm in heavy traffic downtown at night. For one thing, it's less distracting to me - the bright white flashing lights reflect off signs up to 100 yards away. I prefer to use those signs as visual reference of cars approaching from behind at night - I can see the reflections of car headlights, which is helpful in addition to my mirror for keeping track of cars around me at night.