Problems for a Modal Epistemology

I drive a little too fast when coming home from work. As I am about to drive through a particularly tight curve, often an image comes before my mind's eye of a bicyclist just beyond the bend. To avoid the possibility of a collision, I slow down. Other times, as I am looking in my rear view mirror, an image of a police car's flashing blue lights comes to mind. This image immediately causes me to take my foot off the gas to avoid the possibility of a speeding ticket. Here are two everyday examples of my using conceivability as a guide to possibility. Because I assume that these images provide me with a guide to possibility, I decelerate. But why think conceivability can provide me with such a guide? That is, why think that my conceiving of a certain state of affairs shows (or even provides prima facie evidence) that that state of affairs is possible?'1