The reason the internal combustion engine won out is that electric cars had limited range. Their range has gotten better since the 1900s, but is still very small compared to that of a reasonably efficient sedan. What electric vehicles are good at is performance: they don’t need gears, which makes them accelerate at rates previously reserved for Formula 1 cars. That’s a niche market, not something that could replace either rail or gas-powered cars.

Electric cars don’t require gas, but because of their limited range, that’s only useful for commuting and errand-running, both of which are natural strengths of public transportation and walking. The greatest strength of car is in low-frequency trips, i.e. vacations (which electric cars don’t always have the range for) and some shopping trips (which become transit-friendly as soon as the cost of driving exceeds the cost of delivery).

I was in Tokyo for 6 weekdays this summer and not once did I see a traffic jam. There are lots of excellent roads, sure, but the demand for them isn’t that great. So it’s really not about the roads per se, it’s about the right pricing point. Owning a car in Japan is very expensive, so even while many people have them, their cars are small, efficient, and spend most of their time parked in the driveway.

With electric cars, it’s a matter of keeping them expensive enough- by sufficiently taxing carbon-emitting power plants or by charging high taxes- so that a large chunk of the populace chooses public transit. Then our existing roads will be in good condition and unclogged, and there will be money for both autos and transit.

The RER isn’t very useful for intra-Paris travel. Its interstation distances are long, on the order of 2 km; on the RER A, the busiest lines, the interstations inside Paris are even longer, since the line runs parallel to existing Métro lines. It runs as a subway in Paris and some of its closest suburbs, but the LIRR and Metro-North run as rapid transit in some parts of Queens and the Bronx, too.

The main reason the RER has so much more ridership than New York’s commuter rail services is that it serves multiple destinations. New York’s commuter rail is very good at getting people from the suburbs to Manhattan. However, it doesn’t serve people who don’t work in Manhattan, because trains terminate at Penn Station or Grand Central, transfers are inconvenient, and schedules favor commutes into Manhattan over anything else. In contrast, the RER A runs through Paris instead of terminating in it; it serves multiple stations in Paris, as well as La Défense, the region’s most important edge city; it makes many easy transfers with the Métro and the other RER lines.

New York actually has an easier time developing such a system than Paris did. Paris had six train stations, all located outside city center, so it had to build new tracks to connect them. New York can just send NJT trains eastward as LIRR and New Haven Line trains, and only then worry about four-tracking the North River Tunnels, connecting Penn Station and Grand Central, connecting Hoboken and Flatbush to Lower Manhattan, etc.

And, as Alon Levy pointed out, there’s also the efficiency of space issue.

The problem with what you suggest for roads is that they create their own demand. Even areas with low levels of population growth and ample highway construction, like Chicago and Los Angeles, can have clogged roads. In contrast, discouraging car ownership and promoting mass transit can reduce congestion even if there’s relatively little highway building, as the examples of New York and San Francisco readily show.

In addition, the main draw of roads is that the way highways are currently built, they’re cheap. But as soon as they’re in tunnel or elevated, costs rise to almost subway levels; capacity, however, remains far lower. A modern heavy rail system can carry 55,000 people per track per hour. A modern highway can carry 2,400 cars per lane per hour. The multibillion dollar Big Dig projects are multilane, but they don’t have that many lanes.