As I understand it, Hashem allows Himself room, so to speak, to modify His plans in response to human conduct. This is how a person has free bechira to choose to murder someone in cold blood, or not to.
But this applies to premeditated deaths by other people, not to accidental deaths. Accidental deaths are entirely decreed by Hashem, except for cases where a person puts himself into an unusually dangerous situation, in which case it is kind of their fault, and Hashem might decide to let “nature” take its course unless they have extra zechusim to allow them to beat the odds.
However, the definition of “unusually dangerous” is relative to current societal norms, so if it is considered normal in society to go about one’s daily business in the streets, then this is fine even if the rates of traffic accidents are relatively high.
Thus if the traffic engineer’s scheme will only prevent accidental deaths, then the other person is correct that it is all up to Hashem who decrees who will die, and the scheme is pointless. But if the scheme will also prevent deliberate deaths, such as terrorists mowing down pedestrians, then it is worthwhile.