Trees give a nice shade canopy and help reduce the noise level of a an otherwise loud city street, plus they can reduce the heat island effect that's a near constant in our concrete cities

I had a recent trip take me both into Manhattan and the Chicago Loop and it was night and day how loud the one with more mature trees to break up the echoey canyon of concrete and glass was than the other, despite the other having more vehicle traffic at that moment

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.

And they need maintenance

Trees and hedges are mostly safer than concrete and (as someone else pointed out already) can be implemented in a way that does not interfere with visual perception of traffic. Also trees provide shade during summer which is also nice.

Done well they work as traffic calming features, which makes traffic a lot safer

That also goes for installing curbs. I'd argue it's worth doing so, since these hedges also provide a little shade and noise reduction that curbs don't.