Hedges can be low enough that pedestrians are visible. Trees typically have their branches well above car height. So nothing is obscured. Either way, it also reduces noise a bit.
That said, along highways (especially near urban areas), they're usually much taller, with the roadside having a barrier. A cross section:
homes - mainwalk - bike path - hedge - barrier - road (and then possibly repeat but inversely).
You can connect the mainwalks and bike pathes with well-lit bridges underneath the road.
Re: @dan1101@lemmy.world , I'd also like to mention that some hedges can be pretty low-maintenance. Here are some that are especially for roads.
Top illustration is what I'm thinking of.
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