Well... I have no idea how to use 555s , but I know a great deal about the system you are using. (Vehicle Speed Signal)
Its a 5V signal that is connected to a hall sensor on you ABS tone ring. When a tooth goes by the sensor your signal wire is driven low (body ground).
I'm doing somthing similar that uses the vss, but I do all my work on PICs.
My guess is that the hardest part will be getting your timer to know when X number of grounds per second is 15mph. For me, (using a pic) I just set an interupt to count lows and then every second I check an see how many I have... Multiplied by the number of teeth I have, divded by my gear ratio, etc etc I know how fast I'm moving.
I cant be much help with a just hardware approach. I'm dumb and can only figure out processors (go figure).
But it sound very possible.