I-235 freeway at 23rd and Day streets behind Drake park. Photo credit: Tom Gilsenan

This is a Facebook post by Tom Gilsenan. Posted with permission.

SOUTHERLY WINDS BLOW ROAR OF FREEWAY INTO OUR
DRAKE NEIGHBORHOODS IN THE EARLY MORNINGS

That roar from the I-235 freeway has been back again in the Drake area this week. In fact, it has been a regular feature of early mornings all during this fall.

You likely have heard it if you live south of University avenue, especially east of 31st street. It’s particularly strong in the early mornings, before the city awakens for the day.

At times, the wind this week has been strong enough to carry this ‘roar’ across University avenue and into the Drake university campus. Visitors to the campus one morning last week could clearly hear the freeway in the open area north of the library.

This roar doesn’t happen every day or night, so it can often come as a surprise to those living in Drake area neighborhoods. First thought: What’s going on out on University avenue? Second thought: Is there somebody holding a car rally in the neighborhood?

Neither is the case. It’s the sound of the I-235 freeway, amplified by winds from the south and southwest.

By the time freeways came to Des Moines in the 1960s, highway engineers were lowering the roadbeds to decrease the noise. Much of I-235 is sunken for this reason.

Many earlier freeways, especially in California and Florida, were built at ground level. Walls were added later to lessen the noise level in nearby neighborhoods. That helped those close to freeways, but often resulted in louder noise for those living further away.

The federal standard for freeway noise is 67 decibels. Above that, cities and states are supposed to take steps to lessen the impact, especially in urban areas.

But it turns out that the noise standard doesn’t take weather into account. It ‘assumes neutral conditions — no wind and no temperature effects,’ says Darlene Reiter, president of a highway consulting firm. ‘That happens only occasionally,’ she told writer Meryl Davids Landau in an article for Undark magazine.

Nor does the noise standard account for the fact sound doesn’t travel in straight lines. The noise travels in waves, says Mariano Barrios, who works for the Florida department of transportation.

Those sound waves are amplified by wind.

So when the wind blows from the south or southwest, it carries the sound of the freeway into Drake area neighborhoods. It can sound like there’s a racetrack or truck route in your backyard.