Is hearing protection mainly needed for the highway?
No. Hearing protection is needed on
every ride. There are just too many factors involved for anyone, even you, to know for certain at what speed you're going to start damaging your hearing. The helmet you wear, the bike you ride, how windy it is, and how much noise traffic makes all impact whether your hearing will get damaged. Remember that damage is cumulative over time, so it might not seem like you're damaging your hearing now... but in 10 or 20 years you might find that you were wrong and at that point there's no way to reduce the damage.
So, buy yourself an assortment of NRR33-rated foam earplugs, find a style that works for you, buy a big box, then use them
every time you ride. This assortment is a quick, cheap way to get access to a bunch of plugs to test:
https://www.earplugstore.com/foearpltrpa2.html
Remember to change plugs regularly and keep in mind that plugs sitting on a shelf
do become less effective over time. At least that's what I discovered when I compared a brand-new Howard Leight MAX plug against one from the box that's been sitting on my shelf for last 5 years; the newer plug was noticeably quieter than the older one.