I think the traffic is a big part of it. E.g. the noise from I5 in Seattle (where I live) can be heard for miles either side of it, some days worse than others

I think the traffic is a big part of it. E.g. the noise from I5 in Seattle (where I live) can be heard for miles either side of it, some days worse than others depending on weather conditions. I think things like air conditioning and mechanical plant on buildings warrant closer attention too.