Was there any likely reason given for this effect?

'cause no time was wasted (nor any chance of error introduced) trying to>figure out "What does this red light mean?".

But was it based on the specific car (e.g. following drivers react to> the brake lamps on a 2004 Honda Accord sedan ahead of them faster than> on a 2005 Honda Accord sedan ahead of them), or due to a general driving> environment where red rear turn signals are common, versus a general> driving environment where all rear turn signals are yellow?

A little of both. Study protocol is seldom as "real worldy" as comparing> reactions to an '04 Accord vs. an '05 Accord, though there's a ood> argument to be made that there are times it should be. And general driving> environment does factor into it -- the authors discuss this. Their> findings seem most directly applicable to reaction time whenever rear turn> signal color is known. This knowledge can come from observation (observing> driver sees the car use a signal, so knows that a red light from that car> means "brake", or sees an amber lens or bulb) or from environmental> control (i.e., all cars have amber rear turn signals).>

DS