Your night vision is poor or the color of your Solo's light does not work well for your vision. That light would be fine for most riders at much higher speeds than you are indicating. Have you experimented with the the beam width/tightness?
You may need a vision check or experiment with some lense colors that might give you better night vision. I had clear lenses last year and am using yellow lenses this year. The yellow lenses improve my night vision. YMMV.
I have a Light and Motion Vega which is only 85 lumens. Most people that I have talked to that use it like it very much. I don't think it is enough light. However the Princeton Tec Switchback 3 (300 lumens) and Fenix L2D CE Q5 (180 lumens) that I acquired recently can both be used by me comfortably at higher speeds than you are indicating.