The Obama Administration has announced more details about the national electric charging corridor, a program designed to accelerate the adoption of electric cars by expanding the charging infrastructure. We now know the Department of Transportation will create 48 charging corridors running along 55 interstate highways.
When it's fully established, the Department of Transport is hoping its expanded charging infrastructure will cover almost 25,000 miles (about 40,200 km) of highway. The plan calls for charging stations every 50 miles (80 km) along the corridors. Those stations will be identified by standard Federal Highway Administration signs, just like gas stations and rest stops.
Although this push toward electric power is being driven by the Federal Government, it's backed up by a growing list of companies. Along with Nissan and Tesla, who currently provide most of America's charging stations, the list of companies to get involved includes BMW, General Motors, General Electric and Texas-New Mexico Power.
Along with the electric charging corridor, the push to get people behind the wheel of electric cars has extended to state government fleets. Twenty-four state and local governments have committed to the electrification of their fleets, which will bring about around 2,500 new electric car purchases in 2017 alone.
Source: The White House
You are correct that if it takes 20 min to charge a battery then you only can serve three vehicles in an hour... for that port.
What you are forgetting is that the infrastructure cost to take liquid fuel from a bulk tank and distribute it to multiple pumps is not insignificant and can't easily be extended. It's rather a dangerous operation and subject to breakage and issues with permitting and constant environmental monitoring. The pumps are rather large and expensive and require heavy maintenance. The delivery is setup with a small amount of delivery points (as few as 2 at a station and as many as 100 or more pumps at a mega gas station like a Texas Buc-Ees.
We must also remember that with an electric charging vehicle there are two main places that a user would be charging... at home before a daily commute from their own charging infrastructure (maybe fueled by local solar panels) and on the road while traveling an extended trip.
We don' need to concern ourselves in this discussion with charging speed when it's a vehicle we have parked at our home overnight/during the day before a commute. It's only the long trips that charging time concerns come into play.
But, with a slight shift in thinking, folks tend to do two things when traveling a long distance... one is refuel their vehicle but the second is refuel themselves. They will often (usually) stop for a meal or bathroom break of 15-60 minutes duration. The delivery infrastructure for electric refueling can be installed to service any pair or even quad of parking spaces outside of a restaurant. No longer would we need to stop twice, once at a gas station and again at a restaurant as the restaurant can have electric charging plugs in their parking lot. The restaurant may even wish to subsidize the costs of the electric as an advertising leader to get more bodies into their restaurant. The economic model of being able to install electric charging stations anywhere that the electric grid is deployed are far greater than the economic model of needing a fixed central land location, dig pumps and tanks into the ground and monitor the ground for leakage from those tanks, pumps and plumbing.
I'm not suggesting that the cost of deploying an electric charge station (at multiple parking spots) outside of a restaurant is free, far from it. But the primary cost of that facility is the cost of buying the parking space and paving to make it accessible. That's an already accounted for cost. The incremental to add a charging plug and billing system is negligible in comparison to the rest of the fixed costs and a mere fraction of the initial costs of a traditional fuel station.