No, we can start improving car infrastructure right away because, on the types of roads that have guardrails, cars are like 98% of the traffic or more. And the rare roads that have both guardrails and sidewalks should probably have the sidewalks on the far side of the guardrails, at which point improved guardrails also protect pedestrians.
Purely from a safety standpoint, outright prohibiting bicycle traffic on motorized roads would be a massive improvement at the expense of inconveniencing a small minority of people (while improving the convenience for everyone else). I'm kind of surprised more states haven't done it yet. Failing that, it's probably better to also use guardrails to physically separate the bike lanes from the motor lanes, which is yet another use-case for improved guardrails.
Purely from a safety standpoint, outright prohibiting bicycle traffic on motorized roads would be a massive improvement at the expense of inconveniencing a small minority of people (while improving the convenience for everyone else). I'm kind of surprised more states haven't done it yet. Failing that, it's probably better to also use guardrails to physically separate the bike lanes from the motor lanes, which is yet another use-case for improved guardrails.