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March 31, 2009 10:33 PM

What is the explanation for spirals of crack-sealing tar I see on the roads periodically?

Examples here and here.

I photographed these in Golden Gate Park, San Francisco.

posted by garethspor to science & nature (13 comments total)

Well, in the pictures, it looks like it's to seal spiral-shaped cracks.

The real question is what's causing the road to crack in a spiral.
posted by phunniemee at 10:37 PM on March 31

Sounds like traffic light sensors.
posted by ambrosia at 10:42 PM on March 31 [1 favorite has favorites]

perhaps this?
posted by special-k at 10:42 PM on March 31

Those are really sort of strange. I think the tar crew was just messing around.
posted by dhartung at 10:43 PM on March 31

I don't think they're traffic sensors, they seem too haphazard, and they seem to be connected to natural cracks, and these were on a lightly traveled road in a park.
posted by garethspor at 10:49 PM on March 31

and I think if the tar crew was just messing around, we'd see more diversity in strange marks...
posted by garethspor at 10:50 PM on March 31

I've never seen pavement sensors like that, and it doesn't look like the tar is covering random cracks, not something cut into pavement.
posted by Good Brain at 10:57 PM on March 31

Perhaps they fill cracks using a container of tar, and after the crack is filled they have some tar left over. They don't want to let the tar congeal in the container, and they don't want to pour a big blob onto the road. So they pour it out in a thin strip, and they do it in a spiral so they don't need to walk away from the repair area.
posted by Joe in Australia at 1:57 AM on April 1 [1 favorite has favorites]

If you could determine whether they spiral inward or outward . . . Joe in Australia's answer seems plausible but I was thinking the opposite: before they begin sealing cracks maybe the pressure or flow rate needs to build up until they get a "bead" the right size.

/just guessing
posted by Restless Day at 4:20 AM on April 1

I'm pretty sure dhartung has it; road repair crews getting silly for a moment. There's a tar smiley face on the Pennsylvania Turnpike, positioned to be visible from an enclosed footbridge that connects a rest-stop McDonalds to its parking lot.
posted by jon1270 at 4:24 AM on April 1

Joe in Australia has it. It's far easier to clean the machine if it's completely empty. In the parking lot in my apartment, the guys just ran it out in a straight line till they emptied their machine. They just got creative.
posted by nomisxid at 6:26 AM on April 1

Yeah, messing around, or running out a bead of sealant. There'd quickly be a big round hole if the road actually cracked like that.
posted by electroboy at 6:40 AM on April 1

Filling cracks is mind-numbingly boring. Sometimes you've gotta do something interesting to keep your mind from flat-lining. There are no spiral cracks.
posted by JJ86 at 7:36 AM on April 1