Why do bridges make strange sounds when you cross over them?

[PNWRoadgeek]

I don't know about the rest of the forum but sometimes when I cross over a bridge I hear this really high pitched sort of whining sound? It almost sounds like a person screaming to an extent. Might just be an Oregon-exclusive thing, because in other states that I've driven through from what I remember the sound is basically nowhere to be seen... or heard I guess. Though I have heard about other states having this sound in parts that I haven't visited, specifically California.

Does anyone know about what creates this sound? And why do bridges have it?

[US 89]

That's almost certainly the sound of transverse concrete grooving. Most concrete roads will have small grooves in them, which helps reduce skidding and hydroplaning in wet conditions. Which way the grooves go changes how they sound to a driver. Longitudinal grooves (parallel to the direction of traffic) might give a quiet growling sound of sorts. Transverse grooves (perpendicular to the direction of traffic) almost always result in a high pitched whine, which gets higher the faster you drive.

My guess is in Oregon, at least your area, most of your roads are paved in asphalt, but your bridges are in concrete. This was the case in Georgia as well. In Utah, there are entire freeways paved for miles in transverse grooving, bridge or not (and actually, a lot of bridges there are in asphalt even when the rest of the road is in concrete). The state now generally uses longitudinal grooving on any new concrete, but transverse grooving can still be found on several freeways around Salt Lake, including notably most of I-15 in Salt Lake County. The whine is actually noticeably quieter on 15 than it used to be because 20 years of traffic have worn down a lot of the grooving. A good number of city streets have it too, but you have to be going above 35 mph or so for the sound to be noticeable.

[PNWRoadgeek]

That's almost certainly the sound of transverse concrete grooving. Most concrete roads will have small grooves in them, which helps reduce skidding and hydroplaning in wet conditions. Which way the grooves go changes how they sound to a driver. Longitudinal grooves (parallel to the direction of traffic) might give a quiet growling sound of sorts. Transverse grooves (perpendicular to the direction of traffic) almost always result in a high pitched whine, which gets higher the faster you drive.

My guess is in Oregon, at least your area, most of your roads are paved in asphalt, but your bridges are in concrete. This was the case in Georgia as well. In Utah, there are entire freeways paved for miles in transverse grooving, bridge or not (and actually, a lot of bridges there are in asphalt even when the rest of the road is in concrete). The state now generally uses longitudinal grooving on any new concrete, but transverse grooving can still be found on several freeways around Salt Lake, including notably most of I-15 in Salt Lake County. The whine is actually noticeably quieter on 15 than it used to be because 20 years of traffic have worn down a lot of the grooving. A good number of city streets have it too, but you have to be going above 35 mph or so for the sound to be noticeable.

Ah! Ok. I'm pretty sure there are a couple of those "growling" longitudinal bridges in Oregon, the sound is very noticeable specifically when going across the Fremont Bridge in Portland. The sound that the Fremont Bridge produces however isn't really a quiet growl and is in fact quite the opposite.

Most of our other bridges do use transverse grooving, though our sound is pretty quiet. The very old overpasses still use longitudinal grooving with the "growl" from what I know. There can be some funky sounds with those transverse ones though, have you seen those Florida overpass sounds? Sheesh, wild to think that those sounds are imaginable.

[PNWRoadgeek]

I've never heard this personally, and I've been to California multiple times. Apparently, their transverse concrete sound over there is less of a loud whine and more akin to the longitudinal growl. Here's an example I found(Sound is at 0:40):

https://www.youtube.com/watch?v=50m7vqCOnl0

It sounds lower-pitched than normal transverse grooving. Here in Oregon, ours is fairly loud and high-pitched. Apparently, the reason why I've never heard this sound in California is probably because it seems like it's pretty much extinct over there. I'll try and find more of these.

I've never been to Florida, but I've heard their sound as well. Forgot I made a post about it already, haven't really researched about bridge grooving since my last post.

[freebrickproductions]

That California example sounds like what you hear on the newer bridges on Memorial Parkway here in Huntsville. The older ones and the ones along I-565 have a higher-pitched sound to them as you cross them.

[Flint1979]

The Mackinac Bridge makes a distinctive sound when driving over the grated part because the tires of your vehicle are rapidly interacting with the metal grating, creating vibrations that resonate through the bridge deck.

[GaryV]

creating vibrations that resonate through the bridge deck.

And through your stomach and bones. 

[ElishaGOtis]

In Florida, you can actually guess your speed to a pretty good degree of accuracy using the pitch of the hum as you drive across the bridge, provided that the standard specification of grind-in grooves are used.

For instance:
- 30-40 generally gives a A to C pitch
- 40-50 C to D pitch
- 50-55 D to F
- 55-65 F to G
- 65-75 G to A
- 75-80 A to B

This all depends significantly on the bridge itself, what tires you have, and sometimes the temperature. These grooves can also be found at some toll gantries, weigh station entrances, and that concrete test track on 301 before they closed it. I'm sure there's an equation out there to calculate this for more clarity.

Allegedly this has even held up in court once...

[CoreySamson]

I did not know that about Florida bridges, but that makes sense. Interesting! I've always liked the sound of the concrete grooves on Florida bridges. Gives me some good memories of going to Disney World as a kid.

On a similar subject, I also like the sound of going over the expansion joints on the Hernando DeSoto bridge on I-40 in Memphis.