What would it to take to build a motorway with a speed limit of 200kph, on which it’s safe to consistently drive at that speed?
To begin with, only buses would be allowed on the motorway. This has multiple advantages. First, a bus can transport the most people per vehicle, reducing the number of vehicles on the road for the same number of passengers. If 40 people are traveling, it’s much better they travel in one bus than in 10 cars with 4 people each, or 20 cars with two people each, because those 20 cars can collide with each other. Second, buses are more eco-friendly than cars. Third, buses would make the motorway accessible to everyone who can afford a ticket, not just the few who own powerful cars that can reach 200kph.
Motorcycles and scooters are probably dangerous at 200 kph, and wouldn’t be allowed on the motorway. Neither would freight, which probably doesn’t need to travel at such a high speed, and in any case would be unsafe to mix with passenger traffic at such a high speed.
Further, high-speed bus drivers could be professionally trained, with higher standards than can be expected from ordinary people. Somewhat like airline pilots. There could be a high fee for getting the license, like ₹1lac, and a high fine for any violation at speed, like ₹10K. Of course, enforcement would be strict, as with planes.
Drivers would make a legally mandated minimum salary of ₹50K per month, so that they don’t feel the need to work long hours to make ends meet. And, like airline pilots, they’d be required to sleep 8 hours before a drive, and to drive for only 8 hours a day, and perhaps no more than 4 at once.
Maybe existing buses can’t travel at 200kph, in which case we’d reduce the height to match that of a car. This would serve two purposes: first, to reduce air-resistance and, second, to make it less likely to roll over. So, the bus would be no higher than a car, but the width and length would still be that of a conventional bus . So, it would have the same seating capacity as a conventional bus. It wouldn’t have an aisle, since you can’t walk around in a vehicle with a roof as low as in a car. Which means people would board from the sides, like a car. Once seated, they’d make their way to the centre, to let others board. Since there’s no aisle, it would actually have a greater capacity than a bus of the same size. The dimensions could be legally enforced, via a maximum height set to that of a car, and a minimum number of seats set to that of a bus.
There would be a minimum gap between vehicles of half a kilometer. Ramps and exits would be 100km apart, to reduce accidents caused by merging or exiting the motorway.
Rather than a minimum and a maximum speed, there would be just one speed, which is 200kph. As long as you have half a km of gap ahead of you, and good weather conditions and visibility, vehicles would be obligated to drive at 200kph, no less, no more. A big cause of accidents on today’s motorways is the difference between speeds of multiple vehicles. The safest speed to drive on a motorway is the speed that others are driving at. The conclusion of that line of thought is to have all traffic go at the same speed.
There would be strict admission control to prevent traffic from getting too bad. Vehicles would be allowed onto the motorway at each ramp only if the number of vehicles already on that stretch of the motorway is less than the limit of the motorway. This prevents accidents caused by too much traffic, too close a following distance, different vehicles traveling at different speeds, etc. When the number of buses is controlled, each will be able to travel exactly at 200kph, without heavy traffic to slow it down.
There would be only one lane in each direction, to prevent collisions between lanes. Remember that all the traffic will be flowing at the same speed, so you won’t need to change lanes to overtake. So one lane is fine. If there’s more traffic, adjacent lanes will be physically separated with barriers, to prevent collisions between lanes, or other types of accidents involving multiple lanes.
The buses would be self-driving, or at least required to have a lot of safety features. For example, speed limiters with a centrally enforced speed limit (via the Internet or radio or something), which can be reduced when it rains, forcing all vehicles to slow down. Cruise control to automatically ensure a set following distance. And collision avoidance systems. And lane departure warning systems.
You’d have automatic braking to warn if you’re too close to the vehicle in front, or to automatically apply brakes. And emergency brake assist, which detects if you aren’t pressing the brake hard enough and increases braking force. Of course, anti-skid braking to prevent skidding. And electronic brake force distribution to optimally distribute the braking force between the four wheels for maximum braking effect. And cornering brake control to help with braking while cornering. You’d have electronic stability control to prevent skidding and loss of control.
You’d have infrared night vision to see ahead farther than headlights can reach. Or, better, shut down the motorway at night. Adaptive headlights that adjust their angle based on your speed and the curvature of the road. And daytime running lights to help visibility.
All these safety features would be mandatory.
It would be interesting to see some country or state start to build 200kph motorways. These would be far cheaper than high-speed rail, which is often so expensive to be economically unviable. Naturally, making incremental changes to existing motorway design won’t let us increase the speed to 200kph while maintaining safety. Instead, we need to think from scratch, from first principles, so that we have speed, safety and reduced air pollution and energy usage, all at once. In fact, the stringent design above may make it safer than today’s motorways, while being faster.
It’s time someone takes the next step, a step into the future.
 Double-decker buses are probably unsafe at 200kph. It’s also safer to exclude articulated buses, since the flexible joint is another thing that could hypothetically go wrong at speed.