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Who taught YOU to drive?

Letters from readers’ mailbag: message boards

Dina Rudick/Globe Staff

We all like to know what lies ahead, especially when driving. That’s why the state’s roadside message board system that displays estimated trip times to various points — e.g., “Exit 33, 5 minutes” — is a great addition to our daily commutes.

That is, when it works correctly.

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“On Thanksgiving, mid-morning, the Mass Pike heading west was its usual turkey-day parking lot,” Lori Factor-Marcus of Malden, wrote to me.

“Why, then, did all those highway message boards keep assuring us that I-84 was ‘24 miles, 22 minutes’ away (my own example, approximating the displays all along the Pike) when travel time would have been at least three times the information given?”

We covered the ABCs of the state’s Real Time Traffic Management System a few months ago.

The boards work by scanning for Bluetooth devices in people’s cars at set points along a road. A computer then calculates the average travel time it takes those Bluetooth devices to go from Point A to Point B.

But Factor-Marcus raises a new angle: How accurate is the system, and what would cause it to fail?

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MassDOT has 60 message boards across the state, and on average, they are accurate 86 percent of the time, said spokeswoman Sara Lavoie.

Why not 100 percent? For starters, individual message boards go down from time to time due to power or communications issues.

But technical glitches aren’t the only reason a board might display an inaccurate travel time.

When you see a board, it displays the most recent information it has about the cars that have traversed the road ahead of you and made it all the way to Point B, whatever that end point is. But sometimes something unexpected happens, such as a car accident, on the stretch of road that’s in front of your car, but behind the cars that have had their Bluetooth devices checked.

Traffic behind the accident could come to a halt, but the system doesn’t know that because the cars that were ahead of the accident still make it to Point B without having to stop.

It doesn’t take long for the message board system to correct its estimates, as each board refreshes every three minutes based on the newest tracking data it receives. But if you are unlucky enough pass by a message board before it corrects, you’ll miss the cue to reroute your trip.

“[That] lag time may cause the travel time estimate to be noticeably different from what the individual motorist actually experiences after reading the sign,” Lavoie wrote to me.

The malfunction that Factor-Marcus noticed the day before Thanksgiving was something different, though. On that day, “a software issue . . . arose from a previously undetected bug,” Lavoie said. The issue was fixed later that day, and MassDOT is taking steps to prevent it from happening again. (The Real Time Traffic Management System, which debuted in 2012, is still fairly new, after all.)

While we’re on the subject of message boards, I had a few of my own questions.

First, what happens when Bluetooth-toting motorists decide to pull into a highway rest stop between Point A and Point B?

Wouldn’t their pit stops ridiculously mess up the average travel time posted on boards?

“The system would discard any data with a travel time that was not reasonable, such as someone stopping at a rest stop,” Lavoie responded.

My second question: Will message boards display a trip time — say, “20 miles, 15 minutes” — that clearly reflects travel speeds faster than 65 m.p.h.?

I should have known better even than to have asked.

“MassDOT’s policy is not to show travel times that exceed the speed limit,” Lavoie said.

Everyone right?

A Medford reader, who wishes to remain anonymous, chimed in with this terrific question about “no right on red” signs at an intersection: If you see one in your direction of travel, does that mean everyone else at the intersection also has one in their direction of travel?

The quick answer, according to state and federal highway officials, is that every approach leading to an intersection is unique.

Whether a no-turn-on-red sign, or even a stop sign, is placed at a particular approach to an intersection depends on factors such as pedestrian traffic, whether drivers have clear sight lines, and other engineering variables. You’re prohibited from turning only when a sign faces your direction of travel, they said.

Pedestrian safety is the primary reason you’ll encounter a no-turn sign, particularly when there is a crosswalk on the street you’d be turning onto, Lavoie said. Or you might encounter one where railroad tracks lie beyond the right turn. (By stopping, the hope is you’ll notice an oncoming train.)

Where are you least likely to encounter a no-turn sign? On an approach to an intersection where there isn’t a crosswalk after the turn.

“Massachusetts state law does not recognize unmarked crosswalks as legal crossing points for pedestrians. Therefore, there is no legal justification to prohibit these turns,” Lavoie said.

Stop, or not?

“What is the rule regarding stop signs in parking lots, such as Market Basket and Kohl’s?” reader Mary Paris of Woburn asked. “I feel foolish stopping if there is no pedestrian in sight, yet we are supposed to stop at stop signs. Can you set me straight?”

I’ve answered this question before, but as it still raises confusion, here’s the short of it.

Anyone can erect stop signs or create roadways on private property, but most of the time, unless a property owner petitions local officials to make such traffic elements legal, they’re essentially window dressing. Running such stop signs, therefore, won’t earn you an automatic ticket.

Does that mean you should wantonly blow through the supermarket parking lot? Of course not. If you cause an accident, insurance companies will scrutinize your actions. To them, a stop sign is just that.

And if you’re reckless, police can charge you with driving to endanger, a criminal offense, even when you are driving on private property.

I’ve heard of at least one tricky exception to the rule as well.

Boston Police Department officials consider large parking lots, such as at South Bay Center’s, to be “public ways,” and thus police them like streets.

Peter DeMarco can be reached at peter.demarco@globe.com. His Facebook page is “Who Taught You to Drive?” and on Twitter @whotaughtU2driv.

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