I routinely ride through our unlit river valley on my road bike with 2 one watt LED's and have no problem seeing where I am going and ride about 40 times faster than your suggested speed... the lower wattage lights seem more effective when there is no light and this is probably due to how our eyes adjust to dark conditions.

The 10w Nighthawk was $65.00 and I use this for night rides on my mountain bike in areas where there are no lights and it serves me really well for my winter commute as the lower wattage LED's do not work as well due to high amounts of ambient light.

I might also be blessed with better than average night vision as I run lower wattage lights than many of my friends and don't have any problem seeing where I am going.