to expand, adding transit capacity doesn't reduce traffic because we are well undersupplied for (latent and induced) road capacity demand at peak usage. additional supply is swallowed up by overwhelming demand because we're far from market equilibrium. we won't be able to build enough to satiate demand in the near future.
but if you look at it economically, adding transit (or road) capacity is a net positive since part of what drives economic activity is the ease with which you can execute transactions in person. so the main benefit of transit is more economic activity, not less congestion.
so transit funding was never really about materially impacting transit/commute times. but now that we have transit funding, LA and the state are both working on upzoning around transit corridors. for example, sb-50 (fka sb-827): https://www.latimes.com/local/lanow/la-me-ln-housing-bill-tr...
upzoning can actually reduce some congestion but it's not enough by itself. we also need to properly price in good/timely maintenance and the externalities of pollution. we need to greatly speed up the government agencies/bureaucracies that slow down construcion (permitting, inspection, planning review, etc.). we need to reduce dishonest roadblocks meant to stop development by making it prohibitively expensive (e.g., many ceqa challenges). we need to better control rent-seeking behaviors around real estate.
more housing, and a more responsive housing market, means people can move more easily, reduce commute times, and thereby reduce congestion.