enter image description hereA dramatic scene in my story requires a pursuit with a vehicle “threading the needle” in flight between some structures. I need to describe how small that hole would be in relation to a typical roadway. This question establishes the “typical” roadway for flying cars.

First these are not aircraft but ground effect vehicles. While Star Trek, Star Wars, Blade Runner, The Jetsons, and every scifi I can think of has "magically hovering" private transportation, I am trying to design a more reality-based highway system for the flying cars in my world. A rudimentary version of these cars exist today, I am merely needing road engineering for these smaller, more maneuverable vehicles working in open “road” traffic.

Ground Effect Vehicle Highway System should accommodate the following elements:

  • Ground effect vehicles (GEVs) are used instead of true aircraft.
  • A typical vehicle would be extended to 10 feet width for flight and have a body height of 5 feet, running up to 15 feet in length. This is the area it presents to gusts/turbulence. It’s loaded mass is typically 2,500 lbs.
  • The GEVs are powered by hydrogen fuel cells with a limited energy density.
  • The GEVs cruise in ground effect between 2-5 meters over a surface, reducing drag and increasing efficiency.
  • The GEVs normally travel at a speed of 160 kph (100 mph) and can maneuver through 0.8G turns at that speed via RAM air ducting and dynamic body control surfaces, at a cost to fuel economy if speed is to be maintained.
  • The GEVs fly on a continuous route that is made up of any flat surface such as canals, open water, or grassy fairways. Passing lanes are vertical. They drive as normal cars otherwise.
  • Drivers are not left to their own judgement at intersections: collision avoidance assisted by the road itself means only deliberate effort could cause a collision at intersections.
  • Assume the system has been advanced to eliminate the possibility of two cars colliding mid-air at intersections, and also that collision avoidance systems can evade potential collisions when any car approaches without autonomous navigation (detailed below).
  • Clear rules of the road determine what lane you can be in for any given situation (detailed below).

Here is a more detailed explanation of the highway design:

These are NOT aircraft

Principles of aircraft, which generate aerodynamic lift with their plane surfaces, do not apply to air cushion vehicles. While both vehicles rely of a pressure differential across their horizontal control surfaces; the aircraft reduces pressure above the plane, while the air cushion craft increases the pressure beneath the plane. Please watch this RC PAR ekranoplan in free flight and GE flight. Note that it is unstable in the air, but when it returns to GE altitude it immediately stabilizes on the air cushion. Therefore, please avoid answers relating to flight principles of aircraft. Turning, stalling, and most other dynamics of open air flight do not work the same for these vehicles. A high pressure cushion is actively created below the craft, so it can not bank or stall while in ground effect. Therefore assume the engineering challenges of turning and maneuvering at speed have been resolved, and controlled free-air flight is possible at an enormous efficiency cost A Computer rapidly adjusts control surfaces and ducts to hold stabile free-air flight.


The GEVs are designed to be more efficient than traditional aircraft, and this is at the heart of this question. They fly in ground effect, which is the area of airspace that is affected by the surface below the vehicle. This reduces the drag on the vehicle's underside, which allows it to fly more efficiently. The GEVs cannot travel as far as a traditional aircraft on a single tank of fuel and every maneuver costs fuel. The highways are therefore designed to allow vehicles free flight with minimal maneuvering. The system also does not need the cars make costly stop and starts at intersections.

This question only concerns a design for a GEV highway system. In urban streets or irregular terrain the vehicles operate on the ground as regular cars. Click here for a video of the cars I designed during a typical GE flight. Note that this is not urban traffic, it's a highway system which simply expands the suburban area around a business center, like the American Interstate system. Even with the average car boasting 800 BHP, the LH2 economy in this world lacks the energy density to give everyone their own personal aircraft. This is why vertical lanes are not possible.


Intersections are handled the way we currently handle passing situations, by a brief acceleration to overcome the obstacle. My vehicles lift up and enter free space at intersections for brief periods at a large fuel expense.

(A typical intersection)

There is of course a complete civil engineering task to fully design traffic flow in this scenario but assume that is accomplished, and please refer to the items outside the scope of this question at the bottom. I'm focusing narrowly on horizontal lane separation given that only real-world aerodynamic forces are maneuvering the vehicles. Here is the jist of the how highway system works:

  • Roads are not rigidly constructed, they are augmented reality overlays in the driver's field of view. Except for basic emergency markers, all traffic signals, lane indicators, signs, and even commercial billboards are augmented reality overlays on your viewscreen.

Driver view with AR overlay of road and advertising

  • Drivers will never need to compensate for normal wind gusts. "Smart Roads" provide active air movement data to vehicles. If a cross current is detected the road sends the "set and drift" data to approaching cars, which anticipate and auto-correct for most gusts up to 45 mph cross-winds. Obviously the effectiveness diminishes at low speeds, so for this question assume traffic travels at the speed limit.

  • Drivers never “swerve.” Smart roads dynamically adjust their own lanes based on road conditions, such as an accident vehicle or passing ship. If your path will be obstructed for any reason such as an accident or passing ship, your display will project a "hill" ahead and your car simply jumps over the obstruction. Proximity sensors prevent tailgating, so cars can never be surprised.

  • The car fully controls yaw to place the car on the smart road, ascending or descending as needed. Drivers work in two dimensions only. The feel of driving the vehicle is much like a car, with a slight addition of pitch and roll in turbulent air. The road determines their correct altitude until they go "off roading".

  • Exiting the highway is done by pulling out of the main lane of traffic, decelerating, and banking away onto another "road" or off-road area.

  • Cars are smart, but not fully automated. They have proximity-based cruise control, predictive gust correction, and lane-drift correction. Mostly however, the driver determines the lane and speed.

  • There is no commercial freight on this highway but the highway will cross marine freight lanes and other obstacles occasionally. This again is handled by hopping the road over the obstacle in a virtual bridge. Again, cars automatically follow the road and there are no surprises.

  • Turns are no less than 250m inside radius and speed limits reduce to 80mph. (This is a correction - originally wrote “diameter”)

  • There is no anti-gravity anything. They rely on Power-Augmented RAM for lift and real-world aerodynamics.

  • They can use air cushion hovering over water, wheels otherwise. enter image description here Vehicle concept in GE flight

(These pictures are concept art for reference, not a final design)

I do expect the vehicles to still be "road legal", meaning they are no more than 8 feet wide on ground, 10 feet max with extensible wings. They have very basic wheels which can brake and provide urban/neighborhood propulsion.

Today physical road lanes are officially exactly 11 feet wide, for a maximum legal vehicle width of 8 feet.

With consideration for minimizing correction maneuvers: If I were to visualize traffic in this type of highway, what would be the normal lane separation on this highway?

An answer addresses how close we can be to oncoming traffic without disruptive turbulence, and to passing traffic in a lane to either side. I need the highway as narrow as possible without creating an excessive need for correctional maneuvers when other cars interract.

Vehicle performance will be much like the vehicle in this patent, however thrust and yaw can be used to decelerate the car at 0.6g on the road, but not at all in free flight where obstacles need to be handled as a plane would.

Items outside the scope of the question

I am only looking at lane separation for this specific road. As I describe driving on this highway in the story, what does it look like? The following other related engineering problems will be handled elsewhere:

  • Traffic queuing for entering and exiting vehicles

  • Aircraft

  • Left-hand turns

  • Vehicle technical specifications not already included

  • Urban (low speed) travel

  • Smart road fundamentals of operation

  • Weather other than winds (precipitation, temperature, etc.)

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Monty Wild
    Commented Nov 15, 2019 at 3:18
  • $\begingroup$ I added some stuff to chat. $\endgroup$ Commented Nov 18, 2019 at 14:58
  • 1
    $\begingroup$ Out of curiosity, why isn't everything automated? You've already delegated so much to the computer. Positioning of other cars, dangers, how the road should be at that particular time thanks to weather. Automation seems a logical step from there. Just fill in your destination and your car does the rest. Safe speeds, the best road, to even parking spaces. You can have 0 separation and have it as safe as it will ever be. $\endgroup$
    – Trioxidane
    Commented Aug 31, 2023 at 17:16
  • $\begingroup$ @Trioxidane For the reasons explained, as well as the reasons full automation (FA) isn’t on public roads today. FA has been possible since 1960 and FA UAV’s serve military missions, yet we don’t even fully automate freight trains. Humans won’t buy them, 90%+ of us need control. They would also be a huge national security risk - a good hacker can shut down the country or kill thousands of people. A fully autonomous public road just isn’t believable. $\endgroup$
    – Vogon Poet
    Commented Aug 31, 2023 at 20:20
  • $\begingroup$ @VogonPoet automation on railways is actually decently common. A lot of metro systems are fully automated nowadays, & almost all new ones built are. Freight is less common but a railway in the pilbara has it's entire fleet of 100s of locomotives automated now. $\endgroup$
    – OT-64 SKOT
    Commented Sep 2, 2023 at 23:49

4 Answers 4


Something that might be a useful reference is to consider military helicopter separation distance. Because they regularly fly together very close to the ground in dangerous environments, it could give a baseline as to safety. While military pilots like this are certainly better trained than the average driver, you could argue that automation and augmented reality factors probably counter this enough.

What I found, for the example of attack and reconnaissance helicopters, was a distance range of 3-5 rotor distances for close formation and a stagger distance of 150 meters(500 ft). While your craft obviously don't have rotors, you could consider their width as a reasonable enough substitute. This would give a minimum distance of at least 9 meters(30 ft) and a safer distance of around 15 meters(50 ft). Lanes would then be between 12 meters(40 ft)and 18 meters(60 ft) wide(adding the size of the vehicle and giving half the separation distance on each side).

I'm also concurring with the dissenting opinions that this would never work for all of the reasons indicated, but hopefully this helps regardless.

Also, those jumping intersections look suicidal. What happens when the automated system loses power. While traffic light failures are a major annoyance and lead to significant delays, they don't lead to guaranteed pileups at 100 mph.

  • $\begingroup$ Intersection worries: “pile-ups” just can’t happen when the vehicles are in 3-dimensional space (because things naturally fall down). The number of flying cars loosing power during an overpass would likely resemble the number of well-maintained cars loosing power going over bridges today. Assuming that eventually it will happen, and given vehicles will have a predictable momentum, a proper virtual road can align itself slightly left of right to ensure crossing traffic is “woven” together such that IF a car looses altitude mid-passage it will pass between crossing vehicles. $\endgroup$
    – Vogon Poet
    Commented Dec 1, 2019 at 14:54
  • $\begingroup$ But what happens when they fail? That is the problem I don't think you're realizing. It's all well and good to talk about how it will work, but what will happen when it doesn't work exactly as you intend? Most engineering disasters can be traced to this style of thinking. $\endgroup$ Commented Dec 3, 2019 at 5:20
  • $\begingroup$ If the smart road fails at an intersection then the speed limit obviously has to reduce and everyone wastes gas idling along for a while. It would become a traffic cop situation and every car that just lost its road condition feed will alert the driver, which will require by law that they slow down. But yes, if a traffic control blinks out it would become as messy and dangerous as the paved intersections we all love, and someone might crash in those first few moments that the “red light” goes dark. I didn’t solve traffic jams and don’t think I can. $\endgroup$
    – Vogon Poet
    Commented Sep 1, 2023 at 13:12

You're all going to die

There's a saying that applies to GEVs/WIGs as much as it does to hovercraft. If you can see something in front of you you're going to hit it. GEVs are not suitable for traffic situations, as they require a minimum speed rather than a maximum. They're not really suitable for use over land due to the fact they corner like aircraft and as such the inside 'wing' often hits the water. In the case of being over land the inside wing hits the ground and results in something referred to as 'a crash' apparently this is 'bad'.

Basically the way you've designed your transport system, if there's anything resembling traffic at an intersection, everybody dies.

But in terms of your actual questions, we come back to that old, "if you can see it you're going to hit it" problem. Without ground traction you don't have precise maneuvering, without precise maneuvering you must have really wide lanes and be a very long way from oncoming traffic. Possibly in the range of hundreds of meters.

You have no brakes. You have terrible steering. We could consider this equivalent to an icy road and take the 8 second guidelines for safe distances. However the level of automation added to these vehicles means I'd suggest 4 second following distance, 2 second lane width and 2 second vertical distance. For oncoming I'd suggest 4 seconds each way to collision point.

Option 2 is to not allow personal flying transport before full automation.

  • $\begingroup$ If this answer is OK with fully automated ground effect flying cars, then it should be OK with the ground effect flying cars concept? Are you assuming cars slow or stop at an intersection? I can’t tell. That’s not how intersections work. Highways we have today also have a speed minimum, this obviously has that concept carried over. The difference I think you didn’t consider is the fact that our cars can’t go over obstacles while these vehicles can, collision avoidance has a new dimension. Also I have no wings, nothing to touch the ground in banking, but an extendible skirt for slow travel. $\endgroup$
    – Vogon Poet
    Commented Nov 14, 2019 at 14:07
  • 2
    $\begingroup$ @VogonPoet, I live in the UK, traffic can grind to a halt anywhere on any road at any time, there is always a slowing around intersections. The speed minimum is an open road thing, it's more a guideline for practical reasons, for a GEV it's a hard minimum to remain airbourne. You have a lifting body, but cornering still requires roll, that's going to bring you perilously close to the ground while under ground effect. 'Wings' is a useful term to cover the width you need in that body. Without larger wings you have very limited "over" available, both height and duration. $\endgroup$
    – Separatrix
    Commented Nov 14, 2019 at 14:13
  • 1
    $\begingroup$ The trouble is that people on average are barely competent with ground vehicles, you're adding massive complications and crosswinds. $\endgroup$
    – Separatrix
    Commented Nov 14, 2019 at 14:15
  • $\begingroup$ Your last paragraph at least addresses the question but is there any way to put some reasonable number on the lanes, considering there will be a minimum speed limit (to maintain control) and that vertical space can be used for collision avoidance (which will be automated, as stated in the question)? The concept of “swerving” to avoid an obstacle is gone, the car’s proximity sensors used in cruise control just pull up like automatic braking does today. $\endgroup$
    – Vogon Poet
    Commented Nov 14, 2019 at 14:19
  • $\begingroup$ I am adding complications as well as modern collision avoidance, lane sensing, wind correction, and proximity sensing. As the question states, the driver only picks a lane and speed within safe parameters. No different from modern cars which won’t let you smash into the car in front of you, or lock up the brakes, or drift out of your lane. $\endgroup$
    – Vogon Poet
    Commented Nov 14, 2019 at 14:22

It isn't going to work.

Aircraft horizontal separation standards vary based on type of flying (uncontrolled, controlled, VFR, IFR, etc), but is generally measured in miles, not feet. For example, US ATC standards in a terminal area specify a minimum 3 nautical mile separation. Even at Oshkosh during the fly-in they maintain separations of 1/2 mile for airplanes landing on the same runway.

In open airspace, separation requirements can be as high as five miles.

One of the main reasons for these large separations is that aircraft cannot manoever like ground craft, not being connected to a high friction surface. If a vehicle breaks down or suddenly stops in front of you, you're going to need a lot more than 70m to stop if you are cruising at freeway speeds.

The other reason is that the air mass you are flying through is not steady, and the lift of the vehicle in front of you is going to throw off a lot of turbulence.

I know you hand-waved some kind of wind-change alert system, but that's not going to cut it. It comes down to a simple matter of energy - it would be extremely hard to constantly counteract changing winds and turbulence in a way that kept your vehicle in a narrow 'lane'. Even large jets can drop hundreds of feet in wind shear, and it doesn't take much crosswind to make them hard to land.

The logistical problems of many airborne craft trying to stay in densely packed lanes are immense. Which is why we don't have flying cars now, and never will have them in large numbers over any city. Keeping them in ground effect doesn't help and probably hurts, because it keeps those floating vehicles close to many obstacles.

Why do you need this? If they can't stack vertically, just what are you gaining with ground effect vehicles? Certainly not energy efficiency, as these vehicles have to expend power just to keep them off the ground. Small hovercraft are lucky to get 5-10 mpg.

The only reason I can see for it is if somehow the ground can't be used. Otherwise, tires are better.

  • $\begingroup$ To your question, have you looked at the efficiency gains of power-augmented ram vehicles? Energy efficiency is the primary reason. Traction irregularities and tire physical limitations are the next reason. Removing unpredictable road hazards is another (air is sloppy but predictable). Yes, one gain is traded off for another challenge. If the answer is “one mile” then that’s the answer, hoping it’s well referenced however. $\endgroup$
    – Vogon Poet
    Commented Nov 14, 2019 at 21:27
  • $\begingroup$ en.wikipedia.org/wiki/Separation_(aeronautics) $\endgroup$
    – Dan Hanson
    Commented Nov 14, 2019 at 21:29
  • $\begingroup$ But this isn’t strictly aeronautics. That article needs to be dissected to find which factors are and are not relevant at 4 meters altitude. There is no vertical component - or it is trivial at least. $\endgroup$
    – Vogon Poet
    Commented Nov 14, 2019 at 21:32
  • 1
    $\begingroup$ Vertical doesn't matter, Look at it this way: let's say you are in the middle of a 70m lane, and get hit with a 30 mph gust of wind from the side. You are now moving laterally at 13 meters per second, and will be out of your lane in less than three seconds. To prevent this, you would have to accelerate laterally from 0-30 mph in less than three seconds. Good numbers for a sports car on a road, impossible numbers for a ground effects machine already expending most of its energy staying in the air. And 30 mph is nothing. Do the math for a 70 mph gust like you'd get in a microburst. $\endgroup$
    – Dan Hanson
    Commented Nov 14, 2019 at 21:40
  • $\begingroup$ I am aware of these things and I though I said so in the question, the car will never be surprised and can correct for 45 mph gusts (those are actually quite rare at low levels outside a storm). I am working out the (heavy) lateral requirements separately - with many design considerations. For this problem, I'm just assuming those details have been solved and would like to know how to describe such a highway to an audience. $\endgroup$
    – Vogon Poet
    Commented Nov 14, 2019 at 21:46

If all the practical issues are dealt with as you say, I see no reason 'lane widths' would then emulate existing ones, however this is a missed opportunity

One of the major complexities in modern day city / traffic / urban planning is that the 2 dimensional plane, the ground plane, contains:

  • Roads and intersections
  • Buildings
  • Vegetation
  • Rivers
  • Complex topography, perhaps quite steep slopes and mountainous terrain
  • residents with ownership of sky
  • parks, with kids and drones and kite flyers
  • industrial developments, with chimneys, noxious chemicals and exhaust
  • sensitive areas, such as prisons, police and military installations

So there is a fair degree of complexity in laying this all out - without an overlay of free traffic over the top hovering just above the ground plane. Removal of transport from the ground plane will in some ways make things easier, and in others more challenging.

Lane width is perhaps one of the minor issues, but sure 3.5m is standard (in Australia anyway). The restrictive intersections you mention may perhaps also be a lost opportunity to get much more height, and make use of a true 3-dimensional city.

Privacy, security and practicality with regards to the above functions would be another question - but perhaps a great one to ask!

  • $\begingroup$ I’m not entirely sure what this is saying from the wording, am I right to assume you mean that full 3-dimensional roadways would be better? Also, you think 3.5m separation works at 160kph speeds? While most crosswinds won’t push a car out of its lane, the way to compensate for them (automatically) I assume must involve pitching the vehicle into the wind with the rudder and punching the thrust. This means cars can sometimes fly a bit cockeyed in crosswinds, so maybe lanes should consider this? How does flying parallel to another car this close (less than 1 meter) affect aerodynamic control? $\endgroup$
    – Vogon Poet
    Commented Nov 14, 2019 at 15:03
  • $\begingroup$ My reason for avoiding full 3-d roadways is energy cost. I’m trying to leverage ground effect energy efficiencies. I’m hand waving the vehicle design parameters but want the aerodynamic factors fully considered in civil engineering the highway. $\endgroup$
    – Vogon Poet
    Commented Nov 14, 2019 at 15:05
  • $\begingroup$ @VogonPoet ground effect doesn't mean you can't put actual bridges at intersections, or just lose a little bit of energy efficiency for the short while you'll be airborne, even if the car doesn't have enough engine juice to achieve sustained flight $\endgroup$ Commented Nov 14, 2019 at 17:03
  • $\begingroup$ @JohnDvorak I could build bridges, or at least jump ramps, and eventually I might. For now I’m focused on an agile smart road that puts traffic flow priority before making cars fit the infrastructure. Energy cost of maneuvering millions of cars through rigid roads exceeds the cost of the road over time. In this world, the road adjusts to maximize the efficient delivery of traffic demand. $\endgroup$
    – Vogon Poet
    Commented Nov 14, 2019 at 17:18

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .