I haven't seen the methodology but I believe similar tests have been done .... and like @
easyupbug I pay attention myself. I find solid lights are more easily confused with roadside markers or reflectors. Flashing is associated with motion in the human brain, which is sort of hardwired to a response .... as far as I know.
No doubt it is far easier to estimate closing rate with s solid light, but I don't care ... the sooner someone notices me the sooner that person will begin planning not to hit me.