The automobile driving manual says the average driver's reaction time is: .75 seconds....... or 1 car length for every 10mph...... I’m not sure of what that would equate to for a cyclist, but 0.75 seconds is probably all the notice you will get before you are creamed by a vehicle about to turn right or cut you off when the driver doesn’t see you.
Or in the case of group riding: Running into the back of another rider. This is why it is important to always be alert and NO Sudden Movements(whilst riding in a group).
Test your average reaction time..
Be careful this can be addictive.
Click here: Reaction Test