Four Seconds for Safety
There were many "ifs" in the 1.5 second standard perception/reaction time that I used when conducting the analysis of a highway collision. If the driver was paying attention, if the driver realized what he was seeing, if the driver reacted appropriately, if the driver...that's a lot of ifs. For many driving situations it is likely far more appropriate to consider 3 or 4 seconds, not 1.5.
Perhaps the most recognized time dependent perception/reaction consideration when driving is following distance. Everyone should be familiar with the Two Second Rule and the encouragement to extend it to 3 or 4 seconds for anything but ideal driving conditions. It's not that difficult to do and one day you might be glad that you made a habit of it.
Where else could this apply in your daily drive? Let's consider signaling a lane change. If signals are used at all, it is often one blink after the vehicle starts to move toward the adjacent lane, one blink as it crosses over the white line and one blink as it takes up residence in the chosen lane. All that really tells surrounding traffic is what you did, not what you are going to do.
Three to four seconds of signal before your vehicle starts to move laterally will insure drivers around you perceive that something is going to happen, decide how it is going to affect them and if necessary give them time to react and avoid a collision if you are making a mistake.