Natural Gas for trucks.
Electric, Natrual Gas, or Petrol for cars.
Natural gas? So, you're pro-fracking. Cars are more important than clean water. Nothing is good about petrol, either.
Even if it weren't ecologically retarded, natgas is not a viable replacement for diesel. We use diesel for long-haul OTR trucking for many reasons, but not least because of range. You have to make many more refueling stops for gasoline than for diesel, and you have to make even more for natgas.
Now, to be fair, it is possible to make natural gas from an ecologically friendly source — feedlot manure. Currently this is mostly put into holding ponds to sit and stink for a time before being flushed into a waterway. The smart thing to do with it is put it into tanks (or even into glorified ziploc bags such as the ones often used as water tanks now) and let it cook itself, producing methane in the process; it's going to do this anyway, but we can capture it and put it directly into any vehicle with a propane conversion which involves an O2 sensor. You can buy the conversion parts (the PCM, injectors, and fuel rail) straight outta China for around five hundred bucks.
Methane aside, there is also Butanol, a 1:1 replacement for gasoline. BP and DuPont own a company called Butamax which holds the patents for cost-effective commercial production, which were developed partly with our money. GE Energy Ventures owns a company called Gevo which would like to sell it to us, but Butamax sued them to stop production.
As for diesel, the best replacement is "green diesel", which is what you call it when you crack fats (waste or otherwise) in a distillation column such as we currently use for the distillation of crude petroleum. Like biodiesel produced by the transesterification of fats with methanol or ethanol it is a carbon-neutral fuel (assuming you get your conversion heat energy from a carbon-neutral source) but its other properties are more similar to petroleum diesel, like its gel point. It can be blended with 2-10% traditional biodiesel to deliver a fuel which can be used in any diesel engine and, which will actually extend the life of its fuel system with superior lubricity.
The best solution for reducing emissions in locations where it is excessive, assuming we're going to keep driving around like a horde of motorized lemmings, is battery electric. So sad, it only solves the needs of what, is it 80% of the population? Even if were only 50%, that would still be a massive victory.