Matching Speed and Safe Driving

One of the central tenets of my religion is the legitimacy of governments.  As such, one of our behavioral assertions is that we obey the law of the land.  Many years ago, I was in a religious class for young adults.  The teacher asked the students what sin is the least serious in God's eyes.  I, being a little bit of a troll but also being something of a philosopher and wanting to make a point, raised my hand.  When called upon, I suggested that perhaps violating the law of the land, and specifically the violation of speed limits while driving, was the least serious sin.  The point the teacher was trying to make was that any sin, no matter how minor, would keep one out of Heaven unless repented of and forgiven, and the point I was trying to make was that even something as seemingly trivial as speeding qualifies as such a sin.  The reaction of the other students was so completely different from what I expected as to significantly shake my faith in my friends.  Nearly every single student in the room instantly started making up justifications for committing this sin.  One said he was just not ready for such a high law.  Another said she just really enjoyed going fast.  One, who I want to focus on, justified herself by claiming that following the speed of traffic is safer than going the speed limit.  The only person in the room who took my veiled criticism the way it was intended was the teacher, who openly admitted that he struggled to adhere to this law, expressed shame in his sin, and expressed a desire to improve.  I was the only person in the room not bothered in some way by my question, because by that time I had already put sufficient effort into self control while driving that I only very rarely exceeded the speed limit and only accidentally.

This is a nice story, but religion is not the focus of this article.  The one justification that bothered me the most was the claim that speeding is safer than following the speed limit.  It seemed to be true.  For an accident to occur between two cars, they must be going at different velocities.  Speed is an element of velocity, so if two cars are traveling at the same speed, they are more likely to be going the same velocity than if they are traveling at different speeds.  From a religious perspective, this made me wonder if God would justify speeding if it was necessary to remain safe.  From an analytical perspective, I wondered exactly how much difference it would make.  Clearly if you are constantly being passed by other cars, there is greater opportunity for someone to make a mistake that causes an accident.  At the same time though, what proportion of accidents involve vehicles passing?

All of this happened well over 10 years ago.  Since then, I have continued to religiously adhere to the speed limit.  At the beginning of this year, I started attending a college some 30 to 40 minutes from where I live, where I spend most of the driving time on a highway.  What I have observed strongly opposes the claim that matching speed with traffic is safer than going the speed limit.

To start with, I want to discuss the prerequisites for an accident to occur between two vehicles.  I mentioned before that they must be going different velocities.  Velocity is the composition of speed and direction.  Two vehicles traveling at exactly the same velocity cannot ever collide.  They will either be traveling on parallel paths separated by space, or they will be traveling on the same path at different positions in time.  Roads guarantee massive overlap in paths traveled, so on a road, the critical factor is separation in time.  The next prerequisite is that the vehicles must be close together in both time and space.  Again, roads guarantee they will be close together in space but not necessarily time.  Now, to be clear about this time element, when two objects are on the same path in space but not time, this means that one is in front of the other, but they are traveling the same path.  (And the gap in time can be anything from seconds to decades or even centuries if the path is that old.)  An accident occurs when paths intersect in both time and space.

What all of this physics stuff means in layman's terms is that for an accident to occur between two vehicles, they must be close enough to each other that they can easily attempt to occupy the same space (ie., collide) before either party reacts.  When this happens, an accident occurs.  The severity of an accident is dependent on additional factors.  The biggest factor is the magnitude of the difference in velocity of the two vehicles.  A big difference will do more damage than a small one.  Thus, two vehicles going about the same direction and about the same speed will have a far less dramatic accident than two vehicles going in opposite directions at very high speeds.  The other factor is the points on the vehicles that meet, as a front end collision is more likely to destroy the mechanics of the vehicle and harm passengers than a rear end collision.

On highways, vehicles near each other are almost never traveling in opposite directions.  They are generally following the same or parallel paths.  So the direction element of velocity is not an issue.  The difference in speeds are also typically fairly small, with the exception of on and off ramps where some drivers struggle with limiting their acceleration and deceleration to the ramps.  Typical speed differences are around 10MPH (ranging from 5 under to 5 over), which is pretty low for an accident.  If you are going the speed limit, you can expect a vast majority of traffic to being going between 3MPH and 5MPH faster than you are, which is quite low for an accident.  If you are constantly being passed though, this is enough of a difference to increase risk at least a little bit, because cars are constantly changing velocity around you.  In these circumstances, it would certainly be safer to match speed with traffic.

My experience suggests that the circumstances required for an accident are exceedingly rare, when going the speed limit.  If you typically match speed with traffic, you will likely not understand this, because you will constantly be surrounded by traffic, in what is sometimes called a "wolf pack".  A wolf pack is a group of cars traveling fairly close together, and they form quite naturally on roads of limited width where people are traveling at a variety of different speeds.  All it takes to compress a sparsely distributed group of cars into a wolf pack is a vehicle going the speed limit in front of them.  First, the group slows a little, starting at the front and slowly moving to the back.  This allows the cars at the back to catch up, compressing the group front to back.  Then, as they pass, they line up.  Slower cars will pull back into the right lane after passing and faster ones will get ahead, and for at least 5 minutes, most of the cars will stay in a fairly compressed wolf pack.  If, during those 5 minutes, there is another obstacle, the process will be applied again, keeping the wolf pack in formation for at least 5 more minutes.  In practice, wolf packs can maintain a compressed formation for hours, so long as any cars leaving the pack (either going significantly faster or slower, or leaving the highway entirely) are eventually replaced.  If you match speeds with traffic, you will spend almost all of your driving time in wolf packs.

On the other hand, if you don't match speed with traffic, instead going the speed limit, you will be the one getting passed by wolf packs.  All of this getting passed may seem dangerous, but if you have never tried this, what you might not realize is that there are often massive gaps between wolf packs, where you will be almost entirely alone!  This is my experience.  I make the drive to and then back from classes four times a week.  That is eight 30-45 minute drives every week, going at almost exactly the speed limit the entire way.  On average I am passed by 3 to 4 wolf packs.  Each one takes between 1 and 3 minutes to pass, for a total of 3 to 12 minutes spent with other cars around me.  The entire rest of the time there is a gap of around a quarter of a mile behind and in front of me, which is far larger than the recommended 3 second following distance, making me the single safest vehicle on the road!

The thing people forget about the speed matching argument is that when you match speed, it guarantees that you are close to many other cars the entire time.  Being close to another car is one of the prerequisites of having an accident with another car.  If you go the speed limit, you are very likely to spend far more time without any other cars nearby, and the brief periods where wolf packs are passing you do not increase your risk by anywhere near as much as your alone times decrease it.  In short, matching speed with traffic is only safer when traffic is so heavy that it is constant along your entire path.  If there are significant gaps you could be enjoying by going the speed limit, then going the speed limit is far safer than matching speed.

If you want to try driving this safer way, there are some things you will need to get used to.  One is slowing down and traveling below the speed limit sometimes.  When passing, you want to be at the front or the back of the line, and if other cars are going faster than you, it will be hard to get in the front (back is better, by the way, because you have better visibility of hazards in front of you than behind you).  This might involve going as much as 5MPH under the speed limit for several minutes.  This is fine though, because for safety, you want the faster (less disciplined) drivers to be in front of you.

I also want to share a few other observations.  First, in my experience, the most aggressive drivers are the most likely to end up really close to me when I get to me destination.  What I mean is, the drivers that pass me at 10MPH or more over the speed limit have better than even odds of ending up right in front of me when I get to my exit or they get to theirs.  If you are exceeding the speed limit so much that you are constantly changing lanes and weaving in and out, you are almost certainly going to get yourself stuck behind some slow vehicle for long enough for me to catch up with you.  And no offense intended, but I will be laughing at you when I see that all of your speeding saved you no more than a few seconds (while costing you more in fuel).  On some occasions, I even end up passing people who have done this (I also laugh when this happens).  Second, if you have a strong aversion to slowing down, you are prone to drive more dangerously.  A constant problem I see is people pulling in front of other people with around 1 second of following distance.  The average human reaction time is 0.25 seconds.  That means, if the person in front hits the breaks, and the person behind responds as fast as possible, there will only be 0.75 seconds for the person behind to stop without hitting the person in front.  This is modified by quality and condition of breaks, quality and condition of tires, weight of the vehicle, and general road conditions.  While 0.25 seconds is the average, reaction times as high as 0.5 seconds are common.  The recommended following distance is 3 seconds.  Pulling in front of someone with only 1 second of following distance is not just careless, it is incredibly rude, because it also endangering their life.  And pulling in front of a semi with a short following distance is practically suicidal, because a semi requires several times the distance (and thus time) to stop than lighter vehicles do.  If you need to get off at the next exit, you should probably avoid passing, even if it means you have to slow down, and if you are in the left lane, you should try to get behind the guy beside you, instead of hitting the gas and trying to pull across in front.  It might cost an extra second or two, but that's better than costing you your car, weeks in the hospital, or even your life.

The moral of this story is that speeding is dangerous, even when everyone else is doing it.  It has a lot less to do with the speed than driving conditions.  If a particular speed will maintain significant distance between you and other cars, that speed is probably the safest speed.  And frankly, speeding rarely saves significant time.  Also remember that manners have a big impact on safety as well.  When you give other drivers space, you get space too, and that makes everyone safer.  And lastly, matching speed is not safer than going the speed limit.  That myth is a lie made up by people to justify breaking the law, and sadly a great many people have been fooled into believing it to be true.  There may be circumstances where matching speed is safer, but they are fairly rare compared to circumstances where it is actually significantly less safe.