Traffic on the 495 Beltway is backed up from Bethesda to College Park in February 2010. According to a new study, the average D.C.-area driver gives himself two hours to get through traffic for a trip that would take 20 minutes without traffic. (Linda Davidson/The Washington Post)

Researchers at the Texas Transportation Institute, who keep reminding us that the capital region has the worst traffic in the nation, have come up with a new way to measure the pain. They call it the Planning Time Index and rank us most awful by this standard, too.

Planning time is the buffer that drivers think they must build in to assure an on-schedule arrival for priority events. The institute sets a high standard: How many extra minutes you would add to guarantee an on-time arrival 19 out of 20 days.

I frequently see these personal calculations show up in “Dear Dr. Gridlock” letters, and I remember them because they’re difficult to answer. An Annapolis resident tells me she has a flight from Dulles at 7 o’clock 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.

The transportation institute’s index is an overall measure of the unreliability of trips in congested areas.

The PTI for the D.C. area is 5.72, meaning a very 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.

Traffic planners and engineers often refer to the anxiety-reducing add-on as “buffer time.” Dealing with it is becoming more important to travelers.

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 me to reach the meeting in ordinary traffic. 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. In fact, I find it’s the unreliability of trips that bother travelers more then the length of trips. Still, many don’t use the tools available to plan trips.

Few travelers tell me they routinely check online or by TV and radio to get traffic reports before leaving home.

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

In theory: You choose the toll lanes over the regular lanes when you need to be someplace on time and are willing to pay for a reliable trip. The toll rises when needed to maintain a steady flow of traffic. The speed limit on the toll lanes and the regular lanes is the same, 55 mph. What you pay extra for is the steady pace of travel.

Drivers remain somewhat suspicious of this concept, but they’re very interested.

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

What’s your take on the Planning Time Index? 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?

And wouldn’t you also like to see an index that measured the reliability of transit travel? The selling points for rail travel once included the likelihood that you would reach your destination on time. There were schedules.

Many commuters write to tell me they no longer feel they can plan a Metrorail trip with that assurance. And they aren’t talking about relatively rare events like the Green Line shutdown on Jan. 30. They mean all-too-common occurrences such as trains going out of service, or switch problems, or trains too crowded to board.

Dr. Gridlock also appears Thursday in Local Living. Comments and questions are welcome and may be used in a column, along with the writer’s name and home community. Write Dr. Gridlock at The Washington Post, 1150 15th St. NW, Washington, D.C. 20071 or e-mail