Traffic engineers and policymakers are increasingly taking pedestrian needs into account when they set speed limits. By slowing vehicles down, they are improving safety for everyone.
additional note - wiki’s images on fist half of page are mmostly weak wooden or natural aided ones, but lower down there are modern designs. take for example the one over train tracks (though less wide than your highways i am guessing, but take it as an example). They are simple enough, and you would only have to get highway’s permissions mostly to ensure that height is enough (usually highways have a height limit, something like 20-30 feet (2 or 3 storey building)). now most people are not willing to go 2-3 floors, so escalators are common. so “small tunnels” are more convinient. just imagine these bridges, but under ground and upside down. now assuming the beneath of road is reinforced, you just need to dig like 10feet below, and you are good to go. they are more expensive when built after building roads, but still can be built under 100,000
additional note - wiki’s images on fist half of page are mmostly weak wooden or natural aided ones, but lower down there are modern designs. take for example the one over train tracks (though less wide than your highways i am guessing, but take it as an example). They are simple enough, and you would only have to get highway’s permissions mostly to ensure that height is enough (usually highways have a height limit, something like 20-30 feet (2 or 3 storey building)). now most people are not willing to go 2-3 floors, so escalators are common. so “small tunnels” are more convinient. just imagine these bridges, but under ground and upside down. now assuming the beneath of road is reinforced, you just need to dig like 10feet below, and you are good to go. they are more expensive when built after building roads, but still can be built under 100,000