# Planar rail (ConceptTopic, 3)

The planar rail is Keiji's idea for 4D road transport.

The basic concept is for the base of 4D vehicles to be confined to a 2D surface (or in practice, a pair of parallel surfaces, like how 3D railways have two rails, one for each lateral side of the train). By doing this, many analogies to 3D road vehicles can be drawn, and it becomes more efficient to drive than in the traditional analog of a 3D surface with "spaceship steering".

One may recognise Keiji's terms of frontal, vertical and lateral and that in 4D, the usual scenario for an animal is one frontal, one vertical and two lateral dimensions. In traditional 4D land transport, this is still the case, and the existence of two lateral dimensions (as opposed to just one in 3D) causes various effects to occur which make driving more difficult, for example, there is no "left or right", but a whole circle of 360 degrees in which to turn, and the orientation of a forward-pointing vehicle becomes important.

In the planar rail system, the two lateral dimensions are subdivided into one confined lateral and one navigable lateral. In 3D, in road transport the only lateral is navigable, while in rail transport the only lateral is confined. So planar rail combines both. Vehicle doors are located on the left and right of the confined lateral dimension - where they would be on trains - while steering operates in the left and right of the navigable lateral dimension. This already improves road safety as it becomes impossible to enter or leave a vehicle in a direction that is part of the road where other vehicles are driving.

Driving in planar rail is very similar to driving on a motorway in 3D. On main roads, you steer left and right only to select your lane - lanes fill the navigable lateral. All changes in direction are done on the confined lateral, meaning the vehicle is pulled into the correct direction while the steering wheel simply says straight on, like how a 3D train never leaves its tracks (under normal circumstances, anyway). This includes at junctions: one leaves the road they are on, takes a sliproad which bends in the confined lateral into the correct orientation, and then joins the new road they were heading for. Junctions do not require any bridges or tunnels, as the different roads simply pass by each other within the 3D surface of the land.

Vehicles would have wheels in sets of four, so a car would have eight wheels, four at the front and four at the back. The wheels would be roughly cubindrical in shape; one of the cylindrical cross-sections would be rounded at the edges like a car tyre, the perpendicular cross-section would stick out at the edges like a train's wheel. They would be attached rigid to the vehicle in the confined lateral, but the front wheels would be attached via a steering column in the navigable lateral.

A family car could easily seat 13 - four (2×2) front seats and nine (3×3) back seats, analogous to our five-seater 3D cars with two front and three back seats. Like cars, buses could seat a lot more than 3D buses - there would be room for 20 seats on each row, in five sets of four seats with a gap down the middle that would be more than wide enough for people to pass each other. While the average 3D single decker might have about 10 rows, for 40 seats, a 4D single decker with 10 rows would have a whopping 200 seats available.

While there would be a convention on left- or right-handed driving in the navigable lateral, the position left or right in the confined lateral would not matter, so if one were to take one's vehicle to another country that drove on the opposite side of the road, they could simply rotate it 180 degrees so that each wheel was now on the opposite rail to the one it started on, and they would flip between left- and right- hand driving by simultaneously flipping the side in the confined lateral.

Due to the existence of the confined lateral, road signs in a planar rail system would be far more visible. Gantry-style signs could be provided wherever necessary without the cost of having to mount them overhead, as they can span all lanes (in the navigable lateral) while simultaneously being to the side of the road (in the confined lateral) at eye height. Mounting signs to one side in the confined lateral also means that your view of a sign is not blocked by a lorry that you're overtaking, a common problem with non-gantry signs on 3D multi-lane roads.