OMG, the commuting forum moves so quickly this got buried before I had a chance to respond. Anyway, I wanted to add that you would definitely want an activated carbon filter in addition to the HEPA. I think most of car emissions are actually gases--NOx, SOx, CO, CO2, hydrocarbons, etc.--which are nicely captured by activated carbon (except for CO2 and methane) but not so well by HEPA. Diesel exhaust is slightly different I believe; it has the usual array of gases but also very large hydrocarbon particulates, which an activated carbon filter should also capture effectively but may not depending on its design. So, it's usually good practice to pair it with a HEPA-type filter.

Now if you really want to be rigorous (NIOSH certification, etc), I think the average person may find the resulting respirator somewhat off-putting due to bulk, breathing resistance, and appearance. Of course these factors should mean nothing to someone who absolutely needs this level of filtration (multiple chemical allergy sufferers, for instance), but the greater majority may find satisfaction with a design that balances good filtration with ergonomics and aesthetics. For example, I just received my Respro Techno and I'm pretty satisfied with its performance so far. It's not giving me NIOSH-certified protection, but probably close and something that I can wear comfortably for extended periods and through which my voice can penetrate easily (another limitation of typical respirators).