What are those dents in I-90 outside Seattle?

Factual Questions
1

If you’ve driven from Seattle toward Spokane, you’ve probably seen these. Starting around 20 miles outside of Seattle, and continuing for at least 50 miles, there are regularly-spaced dents in the right-hand lane of the freeway, three of them under each wheel. They’re present in both directions, eastbound and westbound, but only in the right lane. In the places where the road has been patched, the dents are missing, so I assume they aren’t serving any useful purpose.

I’m not talking about the perpendicular “rumble strips” carved into the shoulder, which alert you when you’re drifting out of the road. These are inside the lane, carved parallel to the flow of traffic, and unavoidable. They look sort of like this:



 lane
divider               shoulder
   | I I I      I I I | ===
   |                  | ===
     I I I      I I I | ===
                      | ===
   | I I I      I I I | ===  rumble
   |                  | ===  strips
     I I I      I I I | ===
                      | ===
   | I I I      I I I | ===
   |                  | ===

All I can think of is maybe a tank drove over the highway when it was new, and then doubled back. Any ideas?

2

Dowel bar retrofit?

3

Hmm, they do look a bit like the “before grinding” picture, but IIRC the road isn’t made of visible panels there. The dents are closer together than that, maybe 2-3 feet from one to the next.

4

Funny, I was about to start a thread asking this very same thing. They are doing this work on one of the local highways here in PR. They look exactly as described on the OP and on the linked picture. Thanks, it was really bugging me.

5

Funny you should ask, I had this question on my mind this weekend when I drove from the seattle area to yakima over the pass. the only place they appear is on the road surface that is likely to get snow cover. My guess is that they are magnets or some kind of sensors for the plow trucks, so when the road surface is covered with snow they can see what portion of the road they are plowing and keep the truck in the lane. Furthering my guess they are on the outside lane so the plow will move the snow off the road surface on onto the shoulder. Another clue for me is that this stretch of hwy is missing the lane turtles (bumps making divided lines between lanes) and all the reflectors are in recessed diviots, to prevent the plow trucks from scraping them off the road surface.

Just my observations. Now what are the tent like structures (less any canvas type covering) ontop of hwy sign pole type thingies (technical terms here) that are on the west side of the pass?

on edit, they aren’t slotted like the picture linked but more round, like a series of core samples that have been filled.

6

Finally got the answer from my brother, he is a supervisor for the state of Washington DOT. He works in Puyallup and originally wasn’t sure himself. He contacted the someone that works that stretch of I-90, the slots were originally for recessed reflectors. The area gets plowed in the winter and regular glued on reflectors would get scraped off. The state installed the recessed variety about 10 years ago but have not maintained them due to budget constraints. There is nothing magnetic and it wasn’t a rebar retrofit. Sorry the answer wasn’t more high tech.

7

Interesting, thanks! Any idea why they’re laid out like this, though? I’ve never seen reflectors inside the lane, and never this many. Usually it’s just a single track, and they’re either between lanes or along the shoulder, so you drive in between the reflectors - with this layout, you’d have to follow the reflective path like it’s the yellow brick road.

8

Duplicate, ignore please.

9

This question has bothered me for a long time now. I don’t think the marks are from scraped-off reflectors as I’ve recently seen these same marks on I-5 between Seattle and Tacoma. I also have a hard time imagining having six-abreast reflectors in lane.

10

I saw the “dents” the OP is talking about when I traveled to Seattle back in August. I’m going to argue that they’re not for reflectors. They’re in the wrong place, and they’re too close together. And not deep enough.

My first thought when I saw them was that they’re honest-to-goodness dents left behind by steel wheels on certain road construction equipment. I have, in the past, seen heavy equipment that instead of rubber tires, has big steel wheels with teeth. Unfortunately, I can’t find a photo of this kind of machinery. But that kind of wheel would leave just the kind of marks I saw in the pavement. As the OP mentioned, there are two rows of these “dents” in the pavement, and they’re spaced about as far apart as the wheels on a construction vehicle.

Also, as we were coming home from Seattle (eastbound), the "dents’ ended in an area of road construction, which suggests to me that this piece of equipment was driven to the desired location. I suspect they weren’t concerned about the damage to the pavement because that entire section is going to be torn up or resurfaced anyway.

11

Think what you want, I got it straight from the horse’s mouth.

12

I have to agree that they seem oddly placed for reflectors – and I just drove that stretch of road earlier today. I thought maybe they had something to do with traction in snowy conditions?

I wonder if there are other spots that were intended for the reflectors? I’m not doubting your brother, racer72, but I wonder if he and we are talking about the same spots in the road. Recessed reflectors would make a lot of sense.

13

Yeah, reflectors are usually installed on the lines, not in the tire ruts. There are so many of these divots, so close together, for such a long stretch of road, as to make reflectors highly doubtful. A reflector in each dent would make the road surface blinding. (True story: I was riding with my sister one night, returning home from a concert in Puyallup, and we came around a curve in the highway and entered a construction area. There were so many reflectors that my sister literally could not see the road.) And reflectors only in the right lane?

But, again, only in the right lane? The right lane is, of course, where an extremely slow-moving piece of equipment would travel. A construction vehicle with the type of wheels I described would indeed be traveling very slowly. <Forensics hat on> Logic dictates that in ordinary circumstances, the distance involved would make it sensible to load such a slow-moving vehicle onto a flatbed truck, and transport it to the work site that way. My theory that the vehicle was instead driven to the site under its own power suggests that it was too big to load onto a truck – big and heavy enough to leave dents in the pavement as it went. </forensics hat off>

14

I gotta believe that there’s been some miscommunication about which divots are being dicussed. I was talking about this with some Seattle natives last night and no one remembers reflectors ever being in those slots. Plus, the fact that some of the recent I-5 construction also exhibits these marks leads me to think this has to be construction-machinery related.

15

The following answer is speculative, based on the information provided by racer72’s brother:

Consider a snowy highway. Three lanes in each direction, or two lanes for the mountainous stretches of I-90. What’s more efficient, running a plow up one side, then back down the same side to clear the snow off all the pavement, and then repeat both moves on the other side of the highway for traffic running the other direction (or have two plows running simultaneously); or running the plow once up the middle of each side?

Obviously, it’s a lot faster and more efficient to run a single plow once in each direction. But this gives you a single plowed track up each side of the highway, rather than clearing both (or all three) lanes entirely.

It seems reasonable to me that if the plow goes up the middle of each side, the resulting track will basically disregard the original lane markings, and will instead follow the middle track represented by the parallel-to-traffic-flow divots described. In snowy conditions requiring plowing, there won’t be a huge amount of cars on the road, so a single lane should suffice until more extensive work can be scheduled; and even if there is increased traffic in the single lane, you don’t want them going at freeway speeds anyway, so the volume packing the lane will slow things down. And what’s more, the “path” of reflectors is more easily seen below the one or two inches of snow that will still cover them, as they’re recessed in the divots and the plow won’t scrape off every last millimeter of snow above the reflective plastic.

Also: Why would the reflectors be “blinding” just because they’re in divots and there are a few more of them side to side? Just make them a slightly different color and post some signs, and make sure the regular lane markers are distinct, and there shouldn’t be any problem.

Again, this is just my WAG, and I await correction by more informed posters, but as long as we’re just speculating, my description makes sense to me.

(For the record, I drive that highway also, and have wondered about this myself.)

16

I asked a friend who works for the City of Yakima about this and she asked her boss (who used to work for the road dept) and this is what the response was:

17

I think we really need somebody to take a picture of these dents, so that it’s absolutely clear what we’re talking about.

18

Unfortunately I’m now home in Idaho, so somebody else is going to have to do it. I agree that pictures would be a good idea.

I know less than nothing about snow removal equipment, really (I just moved to Idaho!), but my first thought was that if they’re related to that they’re some sort of guide for the plows to stay nice and centered in the right lane. I could be totally, completely wrong, this is a WAG.

19

I’m usually in that area (Yakima & on the I-5) so I’ll try to remember to have a camera with me AND get in a safe position to take a shot. :smiley:

This won’t be happening in the near near future though.

20

You’re from California, aren’t you?