Porosity

The state or property of being porous.

Well, that's not much help. What about “porous”?

1. Having or full of pores.
2. Admitting the passage of gas or liquid through pores or interstices.

There! That's what I was looking for. Traffic is porous if it admits the passage of cars through interstices, or, in other words, if there are enough gaps to allow cars to move around.

Saying “enough gaps” isn't quite right, though, because being porous isn't an all-or-nothing proposition. Traffic can be more porous or less porous, completely porous or not porous at all. I can even imagine defining a numerical measure of porosity, based on the location and size of the vehicles involved. That would be neat; all the traffic helicopters could measure and report on porosity.

I've said that porosity is a property of traffic, but it's not just any property, it's a desirable property. Why desirable? Well, porosity is a direct measure of how easy it is to change lanes, and when you can't change lanes when you need to, it's bad … you get stuck in traffic, or miss your exit, or whatever.

Since I think porosity is a desirable property, I sometimes make an effort to increase it, by letting a car-sized gap open up in front of me. I'm not completely sure this is a good idea, but it seems reasonable. I do worry that by creating gaps I'm compressing the cars behind me, but I also see that other drivers often make use of the gaps I've created, so I must be doing something right.

One might imagine that porosity is an inverse function of density, but that's not quite right. Consider the following situation.

Both lanes have the same density (3/4), but the left lane has some pores, while the right lane has none—at least, none for cars of this size. Thus, although density does place an upper bound on porosity, it doesn't completely determine it.