huh?

July 27, 2011 6:52 AM

why was this police car swerving across lanes?

last night at about 11:30, right before entering the lincoln tunnel on the new jersey side, i noticed a NJ state police car with its lights on to my right. i thought somebody got pulled over, but then the car changed into my (middle) lane, and i though, wow that was unsafe... he should at least put his blinker on. before i could finish my though, he swerved into the left lane. and back into my lane. and back into the right. the car continued swerving, with lights on, for about 10 minutes, then sped up and drove away.

why? was he trying to slow down traffic? is this a common? have you seen it?

posted by sabh to grab bag (18 answers total)

This exact thing happened to me once, in Arizona. It was very late at night, I thought he was trying to pull me over, so I started to slow down, but then he kept swerving back and forth and it seemed to have nothing to do with me.

At the time I assumed he was just fucking with me -- my dad was friends with most of the guys on the local police force. Later I wondered if he was just practicing some maneuver.
posted by hermitosis at 6:54 AM on July 27, 2011

Rolling block. Trying to slow down traffic before a hazard (accident, road work, etc.) ahead. Common here in the SF Bay Area.
posted by mollymayhem at 6:55 AM on July 27, 2011 [11 favorites]

Traffic break.
posted by notyou at 6:56 AM on July 27, 2011

Slowing down traffic to create a gap in the flow. Maybe giving someone time to clear a hazard off the road. I've seen it a few times in the UK.
posted by Leon at 7:08 AM on July 27, 2011

Yes, I've seen that. And my thought was, "Either that police officer is very drunk this morning, or someone has stolen a police car and taken it for a joy ride. I KNOW! I'll get in front of hm just to be safe."

And then the state trooper pulled me over, even though I wasn't swerving between lanes and not using a turn signal.

This was in Richmond, VA, about a year ago. I've had a driver's license since 1990, and I'd never encountered that kind of police maneuver before.
posted by emelenjr at 7:27 AM on July 27, 2011

Yup, rolling stop.
posted by InsanePenguin at 7:33 AM on July 27, 2011

I saw it a while ago when a highway patrol officer needed to completely stop a busy freeway (US 101 in San Jose). I and several motorists had hit a large piece of metal in the middle of the road, damaging our vehicles. While on the side of the road, I watched the officer performing the traffic break. He then stopped completely, stopping all cars on the freeway, then got out, waving his hands in a "nobody move" fashion, and then removed the metal piece. It was amazingly effective.
posted by zsazsa at 7:34 AM on July 27, 2011

I've seen NJ troopers do that same maneuver a few times on the parkway and I've always wondered what the hell they were doing.
posted by crankylex at 7:51 AM on July 27, 2011

thanks, all! i guess s/he was trying to stop traffic, although i didn't witness any traffic ahead, an item being picked up off the road or anything of that sort.

drunk police officer and stolen car being taken for a joyride were two of the first things that popped into my head, as well!
posted by sabh at 8:05 AM on July 27, 2011

This happened to me one night in Connecticut. There was apparently 2 miles of black ice up ahead that had caused 20+ accidents. We sat there on the highway for about 2 hours until crews could treat it.
posted by Jacob G at 8:09 AM on July 27, 2011

Yeah, this happened to me once in NJ and when traffic was allowed to flow freely again we could see that the police had just cleared a large hazard from the roadway (a mattress or two, if I recall). Another time it happened when there was a VIP (the president maybe?) whose motorcade was being given priority (which I only found out later from the news).
posted by katie at 8:10 AM on July 27, 2011

mollymayhem: Rolling block.

Yup, this is (probably) it. I live very close to a freeway, so I've seen this every so often. I can see the accidents down the road, and I'll see police slowing traffic just like that. It's a way for a single car to control a few lanes of traffic.
posted by filthy light thief at 8:15 AM on July 27, 2011

They will run traffic breaks sometimes in bad weather if they feel traffic is moving at an unsafe speed, probably not the case in New Jersey last night, but worth knowing. Sometimes the cause isn't that apparent, as a short break might be to help a disabled car/tow truck get off the shoulder or to make a quick change to a traffic sign. Even fixing a pothole doesn't take more than a couple minutes with quick-setting asphalt. If they were picking up an item from the road, it would have been long gone by the time you got there: that's the point of the break.

I'm not entirely sure why this is such a California thing, but it's definitely a stock part of our highway patrol's bag of tricks. How else would they do it exactly?
posted by zachlipton at 8:19 AM on July 27, 2011

Along with all of the other reasons given here, the rolling break can also help break up a compression wave "traffic jam" further along in the traffic. If the traffic cameras show you've got stopped traffic ahead that happened because the road went over capacity (or someone emergency braked or whatever), temporarily slowing down the traffic feeding into the jam can dramatically increase road throughput over the stop-and-go that would otherwise result.
posted by straw at 9:29 AM on July 27, 2011

The highway patrol does this in California when they want to slow down or stop traffic on a freeway.
posted by zombiedance at 9:54 AM on July 27, 2011

I knew I'd read about this before: traffic waves
posted by SuperSquirrel at 9:59 AM on July 27, 2011

I've seen this quite a bit in northern NJ. State troopers do this a bunch on the GSP to slow traffic driving toward upcoming accidents.
posted by mintcake! at 10:37 AM on July 27, 2011

Previously.
posted by nakedcodemonkey at 12:00 PM on July 27, 2011