Mining, nuclear power generation, improved medical care in the underdeveloped world, even roadbuilding, they all create one or more external problems that affect everyone in a negative way. But as there are negatives, there are also positives for many societal methods and businesses.
What would be the "good" externalities to physically restricting the maximum attainable speed of motor vehicles to 34 miles per hour, or 55 kilometers per hour?
I don't need to give details about the "bad externalities" of speed restriction, as they all revolve around "lost time" and getting from "A" to "B" more slowly. So let me review some of the "good things" that could happen.
First, alternate public transportation that went faster than 55 KmPH would become attractive. A dedicated "bus lane", with commuter buses whizzing by at 120 KmPH while you poked along at less than half that speed would make you more strongly consider leaving your car at home.
Passenger Rail transportation between cities would get a large boost. I regard the current passenger rail system, and especially the so-called, oxymoronic "light rail", in need of a complete rethinking: we still regard a track system that is under-utilized for passenger transport as "properly engineered". The key element to concentrate upon is "axle load".
In Australia, some iron ore rail cars successfully move 60-ton loads in cars weighing 20-tons empty. Potentially, this same track system has the ability to move individual passenger cars loaded with 600 passengers per car. Clearly, freight systems make underutilized passenger systems. They should be separated, and such a separation has long been considered "equal to more than the sum of the parts" (Albro Martin, "Railroads Triumphant").
What is needed for passenger trains is provided by the examples of high-speed roller coasters at amusement parks: a narrow track and individual cars, much smaller than current 20-ton "communal" cars (and those are "light-rail" cars.... ??!!)। Remember, inertia is a major contributor to fuel usage। A much narrower track (especially my two-track Dorrwey system, shown above) means more tracks for a given right-of-way. Individual small passenger cars means higher speeds, higher acceleration. Higher speeds and more tracks per right-of-way area means a higher frequency of scheduled service. Not having to wait thirty minutes between trains, only to have to sit next to a sleeping homeless individual or menacing social misfit would take away the two most significant negatives attached to current "public rail" transportation.
So these two "externalities", bus usage and rail reform, might result from a physical restriction on the top speed of cars and trucks, motorcycles, anything on the roads, to 34 miles per hour. What about "externalities" to the automobile industry itself?
I will here mention two, but they are worth noting. More are out there, many more, but I want to keep this particular post relatively short.
First, engine size will go down, and the casting of smaller engine blocks will consume much less energy in the world of metal manufacturing. Extrapolate this energy savings over other components as well, and the carbon output of manufacturing is decreased. Electric cars can also benefit from reduced battery packs, as traveling at a maximum reduced speed increases their range and reduces the acceleration requirement that hinders the usage of a smaller battery set-up than current performance criteria dictate.
Small engines and small battery requirements creates a wonderful (in my estimation at least-not so if you're a GM or Toyota executive) opportunity to increase the number of car manufacturers. I prefer to see a variety of different shapes and sizes of vehicles on the road, including custom manufacturers of "sheet metal art" as well as other bodywork material. With a 34-mph top speed, I think plastic windshields (wind screens) would become acceptable, as well as car "skin" that would be fabric instead of painted metal and plastic. Consider those cardboard inserts that inhabit new shoes: those are cheaply pressed out, recyclable, compound curves. Compound curved fenders and roofs are an easy reach from those shoe forms. And, imagine covering those with "wet look" brilliant red fabric one day, leopard spot fabric the next! I think "artistry in autos" would receive a big boost with slower speeds, not to mention we could more easily observe that art if it cruised by more slowly.