Tortoise and Hare Effect: why fast driving in cities is dumb


The Tortoise and Hare Effect

I doubt I coined this phrase, but I’m 100% sure I’m not the first person to observe this phenomenon in action: The Tortoise and the Hare Effect.  Surely you know the classic fable, where a slow but determined tortoise beats a hare in a race because even though the hare is faster, the hare stops a lot while the tortoise plods along, eventually reaching his destination first.  The fable is synthesized down to the maxim, “Slow and steady wins the race.” 

Anyone who has ridden a bike or driven a car in a city knows what I’m talking about:  You’re riding along, someone in a car passes you, you catch up to them at a stop sign or stop light.   Where in this story, the Hare stops to rest and relax because he is overconfident, the car stops because it has to wait for other cars.  The fact that this even happens shows the absurdity of having high speed limits and high design speeds in an urban environment. 

Speed + distance = time.

Higher vehicle speeds do allow drivers to reach their destinations faster, thus saving time, but that saved time is only allowed to amass when a vehicle is able to travel long distances without stopping, such as on interstates or highways with limited access points.  In cities, driving fast doesn’t make any sense, because you will inevitably have to stop for traffic control devices and other cars….over and over.  Ever heard the expression “Hurry up and wait”? This is that.

In a city, the distance factor is removed or at least devalued.  Sure, you can drive fast, but all you’re doing is racing to a stop light or racing to slow for a turning vehicle (basically…hurrying up to wait).  Even drivers will notice that on roads with 4 or more lanes, you’ll often be passed by a speeding driver only to catch up to that driver waiting at a  stoplight.  You may even overtake that driver, depending when the traffic light turns green.  Sure, driving faster may allow you reach a green light instead of a red one, but it’s statistically just as likely that you’ll reach a red light before it turns green. 

Even if driving fast in a city does buy you a few minutes, it’s certainly not worth the increased risk you pose to other road users, especially those walking, biking and using mobility aids.  The faster you drive, the smaller your range of vision is, the smaller the window of reaction time you have and the more distance your vehicle requires to stop (also, the more damage it will do if it doesn’t).  It’s objectively and measurably more dangerous in every way to drive fast in a city.  It’s simple physics. That’s why it doesn’t make any sense to drive fast in an urban environment, and that’s why it really doesn’t make any sense to design for high vehicle speeds in an urban environment. 

So why are speed limits still so high in cities, such as Baton Rouge?  That’s because engineers use an arcane and dangerous guideline for setting speed limits and design speeds of roads…

The 85th Percentile (insert ominous sound effects here.)

The 85th Percentile was based on a more than five decade-old study of rural highways in response to the onset and expansion of the interstate highway system. In theory, the 85th Percentile seeks to match the posted speed limit with the speed that most drivers are driving anyway, minimizing speed disparities between vehicles and, thus, the risk of collision. At the time, driving 50 or 60 mph flat-out scared some people, so they drove slower. On a rural highway, driving 25 when everyone else is driving 60 can be dangerous.

But in cities, it’s always dangerous to drive 60.  Consider what the NTSB (National Transportation Safety Board) has to say about it….”Using the 85th percentile speed to set speed limits on road segments may have unintended consequences. Raising the speed limit to match the 85th percentile speed may lead to higher operating speeds, and hence a higher 85th percentile speed.” In other words, higher speed limits lead to faster driving, which leads to a higher 85th Percentile reading, which leads to a higher speed limit, which leads to faster driving, and so on….For more information on how the 85th Percentile works, go here and here

The 85th Percentile has several fundamental flaws that should disqualify its use in an urban environment:

1.       It only counts “free-flowing” traffic.  It completely ignores cars turning on and off roads. This is a problem because cities have lots of intersections and driveways.  This is how you have a speed limit of 45 mph on Jefferson Hwy at Bocage, despite there being heavy traffic congestion there due to all the businesses and driveways.  If they counted all vehicles, not just free-flowing ones, and assigned a speed limit based on the data, the speed limit would be closer to 25, which would actually be appropriate for that environment.  Speaking of….

2.       It doesn’t take into account the surrounding environment.  It’s utilized the same on roadways with schools, parks, churches, bus stations, etc. as roads with long stretches of nothing.  Sure, engineers will tell you they account for those in other ways, but you don’t have to look very hard to find a 40, 45 or 50 mph speed limit next to a school or a park in Baton Rouge.  “Why is the speed limit so high next to that school?” people will ask.  “Because that’s how fast people are driving,” say the traffic engineers.  That’s the 85th Percentile in action.

3.       It doesn’t take into account people who walk, bike and use mobility aids, such as wheelchairs.  Simply put, it’s about moving cars and is not concerned with the effect of high speeds on vulnerable road users.

Engineers will say that simply lowering the speed limit isn’t enough to make people drive at slower, safer speeds.  They’re not wrong.  Road design features have every bit as much effect on driver behavior, if not more, than signage.  This is where advocates and engineers agree. Where we commonly disagree is what to do about it. There are two approaches:

  1. Engineering Approach - use 85th Percentile to set posted speed limit at what speed 85% of drivers are driving at or below. The primary function of the roadway is moving cars with safety coming in a distant second place (IF there’s money in the project budget…IF there’s sufficient right-of-way….IF it won’t impede vehicle traffic, etc).

  2. Safe Systems Approach - Set posted speed limit at what’s safe and appropriate and if people are consistently driving faster, implement design features that encourage people to drive slower, increasing safety. Safety is first and foremost. This approach is better in many ways for many reasons, however…

Engineers won’t even consider slowing traffic on the most dangerous streets, which they classify as “arterials” or “collectors.”  Why?  Because they carry the most cars.   The reason why certain streets need slower design speeds -volume of vehicle traffic- becomes the reason why they cannot have them.  This rationale is self-serving, self-defeating and leads to a circular logic that is rarely challenged from within the engineering community.  This is especially frustrating for advocates because, as we see all the time…


If it did, Baton Rouge, which is full of streets with high design speeds and subsequent high speed limits, would be congestion-free.  According to these same engineers, elected officials and the public in general, this is not the case. In fact, congestion was deemed so bad, that we all agreed to spend another $1 Billion over the next two decades to try and build our way out of it.  Will it work?  That’s another issue for another day.  (But no, it won’t. )

So what can be done?

We have to SLOW. THE. CARS.  We need a comprehensive vehicle speed reduction plan: 

a.       An updated Traffic Calming Manual which allows for calming measures on arterials and collectors and a greater focus on safety rather than consensus and road classification.

b.       Automated traffic enforcement, the proceeds of which are used for three purposes only:

           i.      Administrative/operating costs of the program

           ii.      Supplemental operating fund for law enforcement agencies’ traffic control divisions

       iii.      Implementation of road design changes that make roads and intersections less dangerous

c.       Updated road design standards which begin with safety of ALL road users and accommodate vehicle volume if possible….not the other way around

d.       Discontinue use of the 85th Percentile metric on all roads within municipal limits or at least supplement it with the Safe Systems Approach

 We need our city officials and elected leaders to have the courage and vision to understand that congestion does not result from slow cars; it results from too many cars.  High speeds can actually worsen congestion as they make crashes more frequent and severe and, ironically enough, cause chain-reactive “over-braking.” Now, will we stop trying to accommodate all the cars?  No.  Not in my lifetime.  But we have to be willing to examine decades of past experience when it comes to prioritization of vehicle volume and speed over roadway safety.  Baton Rouge roads didn’t become congested AND dangerous because we were doing things the right way.

There’s no reason we should be driving at highway speeds through neighborhoods.  Even if the speed limit sign says it’s ok, which it shouldn’t, it won’t do us any good.  Slow and steady wins the race, folks.   The humble Tortoise understood that.  Let’s not be like the Hare.  Life is hard enough without wasted effort.  And there’s no bigger waste of effort than racing to a red light. 

Doug Moore, Vice-President