I try very hard not to fall while biking. *rim shot*

Sorry -- what everyone else said -- avoid road hazards and layer as necessary. A bit of trial and error involved -- I found myself habitually over-bundling and roasting myself on a ride before I figured out that it doesn't take very much clothing at all to heat up when your HR is elevated. Rule of thumb I follow is that I should be cold for the first 10 minutes. (This really applies more in the winter, 30s and below.)

Oh to answer your last question -- this will be a matter of personal preference but I start with arm warmers once it's in the 50s and add from there. Also, there's a big difference between 55 and sunny, 55 and cloudy, 55 and wet, 55 and windy, 55 and wet and cloudy and windy, etc. I kept a chart for a while so I could keep track of what worked.