There is a new company promising to build affordable hydrogen cars. The cars are designed for use as city cars with limited top speeds and range. While those limits are likely to be problematic as people tend to buy much more car than they ever need, the company intends to make the designs available to everyone so that they can be adapted and improved locally. This open source arrangement may be novel enough to overcome the speed and range limits, though it's not clear how the open source process will work with all of the regulations to which cars are subject. However, because hydrogen powered cars can be much simpler (meaning fewer parts) than conventional autos (because there isn't a transmission and the engine is electric) there aren't the same economies of scale issues as conventional building.
Another critical feature is that the cars will only be available as leased vehicles. The idea is that companies will design the cars, own the cars, and invest in the required infrastructure. By turning to an open source model with relatively low overhead the fueling network can be installed city by city (or region by region). This seems like a much more promising approach for switching to hydrogen than a national mandate, but at least in the US requires the federal government to get out of the way.