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Speed-Limited Road Design

Physics replaces MPH/KPH signs
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It may be necessary to review the "Evel Knievel Intersections" idea, and also its annotations, before reading this. See link.

Suppose we try using Physics to Enforce a particular speed limit, along some stretch of road. I know already that those who want to go fast will immediately fishbone this Idea, but that is irrelevant to its workability. I think it can be done as described below, but welcome critiques in the annotations. Thanks.

Each direction of traffic needs to be handled similarly, but separately. That's because the physical features described below have to be encountered in a particular order for them to have the desired effect. As a simple explanation of why, consider the result if you built a "speed bump" and a "speed dip" near each other, across all the lanes of traffic: Traffic going one way would experience the bump first, while traffic going the other way would experience the dip first. We need this Idea to work the same way in both directions of traffic.

One of the simplest implementations of this idea consists solely of a "speed dip". It has a fairly steep slope, so that the faster you go when you reach the "lip" of the dip, the more likely your car is to experience the ground dropping faster than the front wheels. Physics is nice and clean and straightforward about the results; gravity operates equally on all shapes of two-axle vehicles.

We design the speed dip such that after dipping down a meter or two, it climbs back up to the normal road level equally steeply. The net effect is this. If a car goes over the dip slowly enough, it goes down and up easily, to reach the rest of the road. But if it goes too fast, the nose of the car crashes into the upward side of the dip! The speeder stops immediately and the Physics evidence of speeding is irrefutable to the Traffic Court. So a nice hefty fine can be added to the fact that the speeder will probably need a new car.

Will the lesson be learned? Physics is inexorable and does not care. But any speeding along that stretch of road will be stopped, guaranteed.

Vernon, Jan 18 2007

Evel Knievel Intersections Evel_20Knievel_20Intersections
As mentioned in the main text. [Vernon, Jan 18 2007]

[link]






       How about the physics variables within the car design:   

       Suspension setup , downforce, wheel size, ground clearance etc.   

       Are you saying that a landrover would be affected in the same way as an F1 car? If so, i don't agree. Design for particular types of vehicles is based on their intended use and physics variables they are likely to encounter. Thats why a Landrover can cross incredibly bumpy ground but an f1 car may spin out and crash if it hit an old shoe.
webfishrune, Jan 18 2007
  

       [webfishrune], OK, I admit I forgot about the interaction between the car and the atmosphere. It is true that a car that has a lot of front-end downforce will be able to go down the road faster than the Land Rover in your example. But it is also true that the average speeder on the average road does not have such a car. Also, remember, I wrote that a speed dip is the simplest version of this Idea. I'm sure other versions can be devised that may be less affected by car/air dynamics.   

       By the way, wheel-suspension is not a significant factor. The dip must be deeper than any ordinary suspension can accommodate, so a too-fast car will have its front wheels separate from the ground, as the first part of the dip drops away beneath them. It is essential that the front wheels not be on the ground, for the car's nose to collide with the other side of the dip.   

       About the ground-clearance thing, I agree that this could be a problem. To be determined in the details, if actually implemented, I think.
Vernon, Jan 18 2007
  

       I see what you mean now, are you suggesting that a car going very fast will attmept to "fly" over the gap and using this effect will impact the upslope on the other side? Interesting idea, I think I understand now. I will withdraw my [-] if that is the case. Having said this however, it would probably only work on reasonably fast roads. I can't imagine this being practical in a 10, 20 or perhaps 30mph limit as the dip would have to be so steep and sharp as to make this impractical.
webfishrune, Jan 18 2007
  

       [webfishrune], the purpose is to ensure that if the speed limit is 30mph, then somebody going 50 will be stopped. I might agree that if the speed limit was 15, then stopping someone going 30 could be more difficult. But I think an alternate design, than a simple speed dip, might still work out.   

       One possibility might be a speed bump with "landing zone" in which a kind of spike is embedded in the road. The faster the car goes over the bump, the higher it lifts above the ground, and the more the ground-impact on the other side of the bump compresses the suspension. Too much and the spike jams into the underside of the car. Very ugly, I know. But workable in theory.
Vernon, Jan 18 2007
  

       You've convinced me. [+]
webfishrune, Jan 18 2007
  

       In my area there is a section of the interstate that is very difficult to drive on because the road surface is very uneven. I dont know if it was shoddy craftsmanship or if it was purposely designed to prevent people from gonig over the speed limit. Its not bumpy and does not have any potholes but as you come across this stretch something feels very wierd which makes it difficult to go over 65MPH while keeping the car in the lane.
Jscotty, Jan 18 2007
  

       // a car going very fast will attmept to "fly" over the gap //   

       When I was young and ignorant, I attempted that once at a railroad crossing. To my surprise, it worked.   

       [-] Interesting idea, but it would be quite detrimental on fuel economy. Looking at the situation more macroscopically, the purpose of speed restrictions is to increase road safety. However I believe "safe" road speeds depend on quite a few factors that are not constant. For that reason, static speed restrictions are a bad idea in general.
ed, Jan 18 2007
  

       Speed restrictions are there to increase road safety. Uneven surfaces, as described, will have the opposite effect (even for drivers within the speed limit).
Texticle, Jan 18 2007
  

       Vernon, I doubt you're surprised to see me here. First let me meet you in your own arena: The physics of the basic idea, in its simple form, is valid. I'm not going to question that. My objections lie in the pesky details which don't matter in a perfect maths/physics environment, but are rather important anywhere else. Your analytical competence is not being questioned.   

       So...   

       If you're looking for a 1m drop, then at 30mph you'll cover 6m in the time the drop takes. In order not to destroy the car on landing (at the end of those 6m it's descending at 4m/s, ie velocity vector at 60 degrees to the horizontal!), you need an equal length of catching space - 6m long, going from 1m to 2m deep, with a parabola returning to the horizontal. Then you need the same pair of features to get out again.   

       For the first 6m, the car is in free fall going into a blind dip; this is lethal. The wheels are touching the road but with no force, they cannot steer or brake. Any obstacle - or pedestrian, they do stupid things at times - down in the dip will inevitably cause a crash. This necessity for downforce at the speed limit means that the parabola needs to be flatter. This pushes the take-off speed up, so the local fire engines won't crash while responding to emergency calls.   

       It also means that speeders won't hit the far wall, but will come flying in with no control, straight into a blind dip and oncoming traffic. That certainly doesn't help safety. /Physics... does not care/ but lawyers do. If you get people killed, or destroy their property, you're busted. You are American, are you not? Thus you understand that designing a road to break people's cars equals mass litigation, even if they were speeding at the time. If they were _really_ speeding, and their car nosed in, it would flip, roll end over end, kill them and also take out oncoming traffic. Extreme situation maybe, but I'm not sure this is a good idea.   

       Then, of course, there's downforce, as already mentioned. If your 1-tonne car is generating 250kgs of downforce then you've immediately got the ability to go quicker safely. This is a problem for any design that relies on the interaction between horizontal velocity and downwards acceleration - anything involving humps or dips.   

       Furthermore, as the road speed gets higher, so the maximum angles designed into the traps become shallower. At some point, a cutoff is reached where the car's suspension geometry means that the wheel will always hit before the nose can, regardless of car velocity. At these speeds the design is worthless - and the speeds fall as the car's ground clearance rises.   

       If I'm not mistaken, [webfishrune] was saying that a Land Rover would eat a simple obstacle like this without pausing for breath, let alone slowing to the limit.
david_scothern, Jan 18 2007
  

       Any design like the roadwave Idea posted will be ripped up in weeks by trailer hitches and flipping motorbikes.   

       As an alternative, how about supersoaker jets on all utility poles?   Forget for a moment motorbikes and convertable autos.   Road surfaces would be continually washed and cars roll safely through at reduced speeds. Use saltwater and kill roadside grass as well.
reensure, Jan 19 2007
  

       I'm afraid, after [d_s]'s detailed explaination I am now unconvinced again and will revert to my original thoughts which [d_s] has convinced me are along the right lines. Halfbaked idea certainly but not practical .
webfishrune, Jan 19 2007
  

       [david scothern], nice critique. Thanks. And it is indeed a good thing that this was not especially intended to be a serious suggestion.
Vernon, Jan 19 2007
  

       [Vernon] Thanks! Webfishrune's vote appears to be travelling through just such a road feature - down, up, down... logically he has to vote neutral eventually, to get back up to his starting level and continue on his route?
david_scothern, Jan 19 2007
  

       Place ridges across lanes strategically spaced to hum out a nice tune from the undercarriage at the indicated safe speed. Drive too fast and it sounds like backward masking .... 'faster .. faster .. faster'.   

       Then put a dip. Folk will remember that for a while.
reensure, Jan 20 2007
  

       "Why are you 6 hours late? And why's your car smashed up?"   

       "I got stuck behind a speeder."
phundug, Jan 20 2007
  

       If you could make the road around the dip transparent (which would admittedly be tricky) that would help the visibility problem.   

       People going faster than the speed-limit using aerodynamics can be punished by hitting their heads on the roof as gravity effectively flips inside the car. The seat-belts might not lock since they wouldn't get a sudden tug like in a crash.
caspian, Jan 30 2007
  

       I get especially irritated by people who drive much too slowly on the freeway. What is really needed in this case is a road along which you can't drive _slower_ than a certain speed. Keep all the scooters, grannies, underpowered vans, etc. off the freeway.   

       My solution: exactly the same as Vernon's top speed limiter, except filled with custard - deep river of gently-flowing custard. Cars that are fast enough can drive over the custard no problem. Slow cars, scooters, etc. get mired in the goop, sink to a safe depth, and are washed into a holding pen where they are hosed down and sent on their way (along a slower road).   

       Come to think of it, don't custard-filled speedbumps count as speed control by physics?
FishFinger, Jan 31 2007
  
      
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