Wednesday, December 09, 2009


On highway exit numbering

There are two ways to number exits on a limited access highway: they can be numbered sequentially, or they can be given numbers that correspond to nearby mile markers.[1] Driving through New Jersey can expose one to both systems: the Garden State Parkway uses the mile-marker system, while the New Jersey Turnpike uses sequential numbers.

Each system has a primary advantage, and each represents a corresponding disadvantage to the other system. When sequential numbers are used, one knows how many exits one has to go, but one doesn’t know how far that is. With mile-related numbers, one knows how far it is to one’s exit, but one has no idea how many exits there are in between.

There’s a secondary disadvantage to the sequential system, which partially negates its primary advantage: if a new exit is added, what number is it given? Renumbering all the subsequent exits is very disruptive, since people rely on persistent exit numbering in printed directions. Such renumbering is sometimes done, posting both the old and new numbers for some years. The less disruptive solution is to append a letter to the number of a nearby exit, as was done for exits 7A (between exits 7 and 8) and 8A (between 8 and 9) on the New Jersey Turnpike. But now, if one is at exit 7 and plans to go to exit 11, one underestimates the number of exits in between by two.

That would seem to say that we should always use mile-marker numbers to number the exits. And, yet, it is useful, sometimes, to have an idea of how many exits one has yet to go.

So why not have a system that gives both? We could have exits represented with two numbers, say mmm-nnn, where mmm is the persistent mile number, which would generally be immutable,[2] and nnn is a sequential number, which might change over time. People would know that exit 132-5 could become exit 132-6 at some point, if a new exit were built at mile 120 (becoming exit 120-5). But when directions said to go to exit 132-5, we’d generally know both how far and how many exits to go.

Good plan? Or too confusing, and something only an engineer could love?

[1] I suppose there could be a third option, giving them arbitrary numbers that relate to nothing in particular, but that’s a pretty silly alternative that I’ve never seen tried.

[2] I say “generally” because it’s possible to do major rerouting of a road that might cause the mile markers to change significantly enough to prompt renumbering. That does happen, but it’s fairly uncommon.


Thomas J. Brown said...

I'm trying to think of an instance where knowing the number of exits before the one I wanted was more or just as useful as knowing the miles to the exit (which is the number I usually care about). I'm coming up blank. But that could be because I've never driven where such a system was in place, so it's not something I'm used to.

If directions of your proposed system were to include both m and n values, I think you'd run into both usability and sustainability issues whenever the n values changed. Using your example: If someone always takes exit 132-5, or if written directions indicate that exit 132-5 should be taken, and it becomes exit 132-6, there could be some confusion. Familiarity with the numbering system should prevent this, and for someone who frequently takes that exit, it's unlikely that a renumbering would happen suddenly and without their knowledge (a frequent user would probably also use geographical cues rather than rely on road signs to know which exit is the one they want). In my mind, it's having to update all of those databases (state and DOT websites, Google Maps, GPS systems, etc) that's the real pain in the butt.

Again, my opinion of this is probably heavily colored by own experiences, but it seems like the m value would be the one to give people (whether I tell them or mapping software does it) and the n value would act as an informational sign for drivers (in much the same way that Google Maps tells you which roads to drive on, but doesn't tell you the speed limit of each of those roads).

Barry Leiba said...

I agree that the number of exits is less important, in general, but it's still sometimes useful. It's especially useful when the number remaining is 1 (you want the next exit), and you can't know that with the mileage alone. It's also very useful when you in or near a city, and the exits are very close together — when you have exits 42, 41B, 41A, and 40 coming up within the next two miles, knowing that you want the third exit may be as useful as knowing it's 41A.

And yes, the idea is contingent upon teaching people that the number after the dash is for convenience and may change, and it's the number before the dash that you use to identify the exit. Familiarity with the system would be important.

Thomas J. Brown said...

Now that you mention it, I do recall driving in Seattle one time and needing to get off at exit 2C, and the direction I was driving meant it came before 2B and 2A, so it would have been nice to know the n value in that case.

Frisky070802 said...

Two comments. One is that knowing your exit is the next exit can be solved by posting a sign that says "next exit 123 in 4 miles". The other is that this is a solution that only an engineer could love. :-)

The Ridger, FCD said...

I like Frisky's idea. Among other things, it wouldn't require a lot of re-signing.

But to be honest, I generally don't pay attention to the number of the exit anyway, just the words.