The eight-minute video has gone viral. An off-duty Boston police officer detaining a pedestrian, after the man struck the cop’s unmarked car's window with an umbrella, when the vehicle turned a corner while the pedestrian was entering a crosswalk. What really caught my eye, though, was an exchange between the man shooting the video and the off duty cop, where they argue about whether the police officer had an obligation to yield to the pedestrian in the crosswalk.

Listen to the exchange:

Listen 0:14 160608-ebh3-lede.mp3

So what does the law in Massachusetts say?

For starters, we might have been thinking about it in a fundamentally flawed way. It’s subtle. But hear how Ryan Caselden, a Cambridge-based lawyer, describes “right of way” in a way I never really thought of:

"The right of way is not something that you have, it’s something that you give. So, you don’t take the right of way, you give the right of way," he explained.

Caselden has taken his share of crosswalk violation cases, so I asked him to explain what the law actually says when it comes to pedestrian and motorist “right of way” in a crosswalk.

It's all spelled out in the General Laws of the Commonwealth of Massachusetts, in Chapter 89, Section 11 and Chapter 90, Section 18A, which together detail both the rules of the road and the penalties for not following them.

From the get go, we see that not all crosswalks are equal under the law.

Chapter 89, Section 11. When traffic control signals are not in place or not in operation...

So at an intersection like Memorial Drive and JFK Street near Harvard Square, where there are walk/don’t walk signs, they rule the day. Pedestrians can only cross when they have the walk sign and must yield when they have the don’t walk. Absent walk/don’t walk signs, if there are traffic lights, they determine when pedestrians can cross and when they can't.

"If you are on the road the traffic signals are intended to conduct your mode of transportation," said Caselden. "Regular car traffic, pedestrian traffic, cyclist traffic, all these other modes of transportation."

So what if a pedestrian goes on a don’t walk, when you have a green, and crosses your path when they technically should be yielding to you?

"You have to stop, you’re required to stop. The idea that anybody would go based solely on the traffic signal is not the idea," said Caselden.

Now there are, of course, no shortage of crosswalks where there’s no traffic signals or stoplights. And it’s at these kinds of crosswalks where the law really favors the pedestrian. Caselden explained some of the gritty details of the law:

So what if a pedestrian is on the sidewalk and look like they want to cross? Here, the law is actually a little less specific, but Caselden says his read is that drivers need to hit the brakes.

"Basically that’s my understanding," he said. "If they have the intent of crossing within a reasonable distance of it then yeah you gotta stop."

There are additional details in Section 720 of the Code of Massachusetts Regulations (CMR), and many cities and towns also have their own rules in addition to what's spelled out by state law and in the CMR.

And Caselden points out that, crucially, these laws only govern whether or not you are liable for a ticket – up to $300 for failing to yield or a $1 for jaywalking (a second jaywalking offense will cost you $2). In the event of an incident or accident, one of the parties can – and often does – sue the other.

"You don’t have to violate Chapter 89 Section 11 to be liable for an accident in a civil lawsuit," said Caselden.

Liability in those civil suits are judged by a whole different set of laws. They’re heard in front of a jury who determine a percentage of liability - say 80 percent driver fault, 20 percent pedestrian. And in Caselden’s experience, in these cases drivers are almost always facing an uphill battle.

"It is hard to be a motorist who's involved in an accident not to have some type of liability," he said. "Stop. No one has ever been on the side of a motorist in a motorist-pedestrian accident – or rarely."

Now if this all seems a little detailed and convoluted, hey, that’s the law. But Caselden says that in reality, following the law is pretty easy: Just let caution be your guide.

"The whole system is based around safety and courtesy, but more so safety," he said. "Everybody on the road – pedestrians, cyclists, drivers – everybody has a duty to use reasonable care and caution when they are using the road in any shape or form."

So be good to each other out there on the roads. And remember, if the right of way is not something you are entitled but something you have a responsibility to give, being safe also means you get to be generous.