I-395 traffic
Many of these drivers on I-395 have probably made a personal calculation about travel time delays. (Alex Brandon – AP)

As if you needed any help, researchers at the Texas Transportation Institute who provide guidance on how bad congestion is getting in urban areas have come up with a new way to measure traffic pain. They call it the Planning Time Index. In the study the institute made public Tuesday, it shows the D.C. region ranking No. 1 in this type of discomfort.

All pain is personal, so you probably have your own name for the phenomenon, but what the planners are trying to describe is the buffer time that drivers build into their trip calculations. It’s not purely a measure of congestion, where the D.C. region also ranked No. 1 in the nation. It’s a measure of the unreliability of trips in congested areas.

The Planning Time index illustrates the amount of extra time needed to arrive on time for high priority events, the institute said in a statement accompanying the new numbers. Such an event might be a flight to a 25th anniversary cruise leaving from L.A., an appointment for oral surgery, or your wedding. It’s a bit more drastic than just being late for work occasionally, unless your boss has you on probation.

In fact, I frequently see this calculation show up in “Dear Dr. Gridlock” letters, and I remember them because they’re the most difficult to answer. It’s the person from Annapolis telling me she has a flight from Dulles at 7 p.m. on a weeknight, so what time should she leave home? I’m very conservative in answering such letters, to the point where the traveler would reach the airport in time to order a full meal and a custom-made suit.

So the PTI does not measure ordinary time. It’s not the slow traffic drivers encountered Tuesday morning at the 14th Street Bridge, or on I-295 or on the Capital Beltway’s outer loop in Silver Spring. It measures how good you are at expecting the unexpected based on how bad your trip can get. It’s how many extra minutes you would build into a trip to guarantee an on-time arrival 19 out of 20 days.

The PTI for the D.C. area is 5.72, meaning a prudent traveler would allow almost two hours for a highway trip that would take 20 minutes in light traffic. Want to travel faster and calculate less? Consider moving to Pensacola, Fla., where the PTI is 1.31.

When I talk to traffic planners and engineers about the calculation drivers make, they often refer to it as “buffer time.” Dealing with it is becoming an industry in American transportation.

On a small scale, you can see this when you ask Google Maps for directions. The results show you the miles for the trip and the amount of time it would take in ordinary traffic. A separate listing shows you the amount of time the trip will take in current traffic conditions.

When I talk to civic groups, I like to tell them the amount of time Google Maps said it should take for me to reach the meeting in ordinary traffic conditions. It always gets a laugh. Then I tell them about the other listing for current conditions and start talking about “buffer time.” Everybody gets it.

But on a grander scale, the reduction of planning time or buffer time is the key selling point for the high-occupancy toll lanes now on the Capital Beltway and coming to I-95 in Virginia in 2014.

In theory: You don’t drive the toll lanes every day. You choose them over the regular lanes when you need to be someplace on time and are willing to pay for a reliable trip. Drivers remain somewhat suspicious of this concept, but they’re very interested. It’s a frequent topic in my online chats each Monday.

The issue we discuss most frequently isn’t whether lanes with variable tolls should exist, but how and when to use them. Drivers want to know how to make the cost-effectiveness calculation on using the lanes when they reach their decision point, the point where the toll rates are displayed on a message board.

My advice is that they should spend a few bucks on some tests. Take the express lanes a few times, see if they actually do save time compared to the regular lanes and calculate the average time savings. Would that quicker and more reliable trip be worth it on a day when you have to reach a key meeting at your office in Tysons? If so, then on the day of the meeting, you know when you walk out your front door that you’ll be using the express lanes.

I think these sorts of time-is-money calculations are going to become a bigger part of commuting, whether measured by a nationwide index or by the price of using the Beltway HOT lanes at rush hour.

What’s your take on the PTI? You probably use a less extreme measure for your own commute, but I’ll bet you do some sort of calculation involving the value of being on time and the annoyance of being early or late. What highways are the least reliable on travel time?