Well.... for policies there should be:
1) Something that deals with maximum loads permited on trucks. There are many laws in real life dealing with that (and te subsequent cost of road repair, where heavy/overloaded trucks cause the asphalt to slide and crack) that could either be taxed or preferably just regulated and would play nicely to your truck/car ratio simulation.

2)there could be some kind of "door dash delivery companies" tax, you know those that deliver food/packages to your house. This could play nicely to the vanilla "allow delivery drones" decision/event and might force the delivery companies into employing more people on bikes instead of car deliveries.

3) To boost money gain from the mod there could be some sort of simulation ( or just add it to the vanilla one) for personal car ownership that boost self-employment and a tiny bit of GDP. It would take into account that people who own small companies (plumber or something) need cheep cars. IT could also play into the one of the most op things in the game, that is rural development grants, which already increase car usage, but they might increase this as well.

4) there should definitely be some kind of law (mabye that "step by step" increase one) that deals with road bariers (i mean those at dangerous places which prevent cars from going off road) and their endings. There is currently a push in europe to stop using the obsolete "just burry it into the ground" ending and transition to the american "crush and throw by the side" ending.
This policy would be interestin in having two "bars" affecting car crashes,just like the "prison regime" already has, one would be a function of "car usage" and the other one a function of "truck usage" (meaning that with increasing car/truck usage the effect would increase) but the truck one would only start working on the high levels of funding. Because trucks need those 2 barriers high, strong support with cable, on both sides in the middle of the highway, expensive bariers for the barier to be effective. It would have to have some kind of exponentional function with a base negative, which then has a 0 minimum so it doesn't get negative at low funding.

5)Another law to raise more money could be a "what type of road is taxed" law. We already have tollroads and satelite tolls laws, this could be a multiplier to those. You know, make only the best highways tolled ==> make all roads tolled. It would work best with satelite taxing, as it would increase it's cost only a little, while the cost of standart toll roads would increase a lot. The revenue from both would increase at the same rate.

6) you could introduce several "punitive taxes" on car ownership.
-We already have a "limit cars in cities" we could make it taxable to enter the city (or add a complex function to the base game policy, which maxes revenue in the middle and approaches zero on the edges).
-You could make a "punitive car ownership tax" where obviously you tax wealthy people with multiple cars. You could theoreticaly have it tax people without cars on the very left, making it a "no car ownership tax", you know.... one of the "despicable ones" like mandatory micro chiping but less overtly evil.

7)You could make "park private ownership" a thing. Where the green public spaces in cities are transformed into gated "gardens"....with an entrance fee of course and the cycling paths could be under that law as well. This would not give you an income (as making milions of $ on that is kinda a stretch) but could have a hefty "privatisation payout" the same way you get money for discontinuing a state railway or water company. It would also increase the use of cars, as people travel to national parks or to the country to visit nature.

8) you could make "non-localy produced food" tax, which would add (a small) tax on food that is not grown localy. It would interact with the city farms and vertical farms policies, where the income would be reduced heavily (like to only 10%) if both are in effect. Mabye even import tarrifs would affect it, as you would probably not tax imported food twice.

9) there could be a "ground transportation tax" which targets businesses that lease cars to people (which exists). altho i dont know how you would calculate that mabye it would need a simulation, where as car ownership decreases, car lending increases ?somehow? You know, people travel somewhere by train/plane and then want to ride the car from the airport to their destination or use it to sight see and such. From what i gather people do that quite regurarily.

10) we already have driverless cars law, mabye shoe in a driverless truck law? it would lower the unemployment reduction (i think it is possible with a multiplier) that the trucking simulation has and would increase productivity and industrial automation (allowing you to tax it more).

11)you could make a "mandartory municipal infrastructure tax"....which is a kind of municipality tax, that is levied on top of your income tax depending on the municipality you live in. This one would be specificaly for transportation, would piss off everybody and decrease the cost of road-building and rail subsidies/company. Basicaly any thing the government pays for in the transport infrastucture part, can be shifted to the municipality budget, and have them deal with it. You could even put in a slight "buerocracy reduction" thing in there, but it has to be small and multiplied by the ammount of specificaly transport subsidy related laws, just enough to counteract them.

12)You could make a tax for "overly long trains" that supposedly disrupt passanger travel, it would get you some revenue, decrease train usage, make comuters happier decrease productivity and piss off the capitalist. (this is also a stretch, as while it does happen in the USA it's kinda complicated for a game)

I could probably go on and on.... but as you can see, past the number 4 the things are starting to get removed from reality and hard to make. So i am going to stop at a dozen.