Talk:Stopping sight distance

Tables

The values in the table don't match the equation. I don't know if the equation or the table is correct but if the formula is correct the higher values of speed should have stopping distances in the thousands.

69.54.143.208 06:01, 15 March 2007 (UTC)Theodore

I have removed both the table and the formula. Obviously, for such a function with so many variables, a simple quadratic seems hardly admissible for application for every type of car. I have added a note regarding this. --Ķĩřβȳ♥ŤįɱéØ 06:07, 15 March 2007 (UTC)

I just got a regression using the data starting at zero and incrementing until 70. It is Y = 0.06X^2+1.1X.

69.54.143.208 06:09, 15 March 2007 (UTC)Theodore

I found a page that has info that says: d = 1.47 V t where: d = brake reaction distance, ft V = design speed, mph t = brake reaction time, sec and d = 1.075 V^2/a where: d = braking distance, ft V = design speed, mph a = deceleration rate, 11.2 ft/s^2 at http://www.ecs.umass.edu/baystate_roads/technotes/35_stopping_sight_distance.pdf

69.54.143.208 06:16, 15 March 2007 (UTC)Theodore

The stopping sight distance table isn't intended to represent a real vehicle. The deceleration rate was obtained by measuring drivers, not vehicles. It was found that most drivers will brake at a rate of at least 11.2 ft/s². Almost all modern vehicles can exceed this rate, but for design purposes, it makes sense to be conservative.--Triskele Jim (talk) 17:00, 5 June 2009 (UTC)

These equations are all fine and good, but there is something missing and/or mis-leading in this article. In the equation above, d = 1.47 V t, if you want t(time) to be in Seconds, and you want d(distance) to be in feet, then 'V' cannot be "speed" in miles per hour...it must be "Velocity" in feet per second, which is why they are using 'V' in the first place. (A long-time frequently used equation in Accident Reconstruction, for example, is D = V x t. See the similarity?). To convert "speed" (mph) to "Velocity" (fps), you can simply multiply any speed by 1.466... or you can divide any Velocity by 1.466. So, for example, if your roadway is posted at 50 mph, and you're traveling at 50 mph, you are also moving at about 73.3 feet per second (fps).

If you want to be really precise, you may multiply any speed by 5,280 (the number of feet in a mile) and then divide that product (answer) by 3600, which is the number of seconds in 1 hour. Hence, the 1.466 "conversion factor" is really the result of dividing the number of feet in a mile by the number of seconds in 1 hour. So, doing it that way, 50 mph is then calculated to be 73.3333...(continuing decimal) feet per second (fps).

I came in after the Tables, mentioned above, were removed. But the Velocity vs. Speed issue may have been the problem and why they did not make sense. I would certainly be open to feedback as to whether I am correct...or not. 2601:1C2:1C7F:5730:4046:4E0F:90C1:B1CA (talk) 03:50, 9 February 2024 (UTC)

Vehicle mass and braking distances

Vehicle mass actually has less effect on braking distances than most people suppose. It actually drops out of the equation. Where

a = Acceleration (or in this case, deceleration) rate

F = Force applied

μ = Coefficient of friction, also called the "drag factor" by police crash investigators

g = Acceleration due to gravity

m = Vehicle mass

N = Normal force, or, in this case, weight

From Newton's laws of motion and neglecting air resistance:

Acceleration rate = mass / force

a = m/F

Force = coefficient of friction * mass * gravitational acceleration

F = μN = μmg

Therefore, acceleration = mass / mass *coeff. of friction * gravity

a = m/F = m/μmg = 1/μg

Mass comes back into the equation when designing brakes and tires. Brakes work by converting kinetic energy to heat energy. The more massive a vehicle is, the more energy the brakes must convert. Put big enough brakes and sticky tires on a semi and it will brake like a Porsche. The problem is, they would probably take up valuable cargo room, taking away from the purpose of a truck - moving cargo. --Triskele Jim (talk) 17:00, 5 June 2009 (UTC)

Definition of stopping sight distance

A completely different definition than the one currently given in the lead is given by this book: J. G. Schoon (2000). Geometric design projects for highways: an introduction. ASCE Publications. p. 27. ISBN 0-7844-0425-9. That definition is:

Stopping sight distance for a given design speed is the minimum distance that a vehicle moving at the corresponding running speed will require to come to a safe halt. It is the sum of the distances traveled during the driver's brake reaction time and during the braking of the vehicle to a stop on a wet pavement.

It appears to me that both this definition and the one in the lead, ascribed to the AASHTO, are somewhat confused; they give the two sides of on inequation between two distances that, for safety, needs to be satisfied. On the one hand there is the sight distance, which is how far a driver can see ahead and spot a stationary condition requiring stopping, such as a boulder on the road. This is independent of the driving speed and the condition of the road surface (except for possible dust clouds on unpaved roads), but may depend on atmospheric conditions. Then there is the stopping distance, which is the distance a vehicle travels from the moment a condition requiring a stop comes in sight until the vehicle actually comes to a halt. This depends on lots of things, such as the reaction speed of the driver, the speed of the vehicle, the condition of its brakes, the condition of the road surface, and the slope of the road. Safe driving requires that the stopping distance always be less than the sight distance. In determining the safe speed limit for a given road, or designing a road for a given speed, assumptions must be made for the reaction speed of a driver and how much a vehicle can decelerate, using safety margins, while the minimal coefficient of friction under realistic conditions may be assumed known. Then it is possible to relate a safe driving speed to a given sight distance, or a safe sight distance to a given driving speed. On a one-lane road with two-way traffic, or a two-lane road where vehicles may use both lanes, the stopping distance must even remain below half the sight distance. I realize that the preceding is all "original research" on my part, but I should hope that there are reliable sources out there that manage to avoid the confusion by giving both sides of the inequation.  --Lambiam 14:09, 26 June 2011 (UTC)

One way to handle this is to make a distinction between "available stopping sight distance," which depends primarily on road vertical and horizontal curvature, and "design (or desirable) stopping sight distance," which is based on the prevailing speed, and provides enough visibility so that "the sight distance at every point on the highway should be at least that needed for a poorly performing driver or a poorly equipped vehicle to stop within the available sight distance." (AASHTO Guidelines for Geometric Design of Very Low Volume Local Roads, page 30) --'Triskele Jim (talk) 17:01, 28 June 2011 (UTC)

Proposed renaming

The following discussion is an archived discussion of a requested move. Please do not modify it. Subsequent comments should be made in a new section on the talk page. Editors desiring to contest the closing decision should consider a move review. No further edits should be made to this section.

The result of the move request was: Not moved - lack of consensus to move. Apteva (talk) 01:34, 3 October 2012 (UTC)

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Stopping sight distance → Roadway sight distance – Request made 16 March 2012 by user:Triskele Jim using template:movenotice. Reason given by Triskele Jim is "Since this article discusses stopping, decision and intersection sight distance, I propose moving it to something like 'Roadway sight distance'." -- PBS (talk) 17:24, 8 September 2012 (UTC)

Since this article discusses stopping, decision and intersection sight distance, I propose moving it to something like "Roadway sight distance" --Triskele Jim (talk) 17:01, 16 March 2012 (UTC)

While I won't oppose such a move, I think that the title "Sight distance and stopping distance" will actually be better description of the topic. I'm more concerned about the content of the article, though, than about its title.  --Lambiam 10:49, 17 March 2012 (UTC)

• Weak oppose. If this is the actual term used, then it is at the correct article name. Now, if more material is covered in the article does that mean that this should be split rather then renamed? Roadway sight distance to me comes across as a meaningless term. But them I'm not an expert in this area. Vegaswikian (talk) 01:46, 22 September 2012 (UTC)
• Oppose, on WP:COMMONNAME grounds. I think "Roadway sight distance" only gets 117 hits on Google Books, and some of those appear to be false positives; "Stopping sight distance" gets 12100 results. I might be open to some alternative name if it was commonly used by sources. bobrayner (talk) 14:44, 23 September 2012 (UTC)
  • I don't think WP:COMMONNAME applies here, since "stopping sight distance" applies to only one of three concepts discussed in the article. To me, the question is whether to move the article to a new name, or split it as Vegaswikian suggested.
    • Would "Sight distance (road)" be better?--Triskele Jim 16:34, 24 September 2012 (UTC)
  • From the limited comments, splitting would seem to be the best option. Vegaswikian (talk) 19:08, 27 September 2012 (UTC)

The above discussion is preserved as an archive of a requested move. Please do not modify it. Subsequent comments should be made in a new section on this talk page or in a move review. No further edits should be made to this section.

Globalize

All the references are from US sources. Other nations may use different values for deceleration, object heights, etc. For example, a documentary I watched years ago said Germany uses 0 mm as the object height for stopping sight distance on the Autobahn, since a pavement defect can cause a crash at those speeds. I don't have anything I can use as a reference, though. --Triskele Jim 16:14, 16 October 2012 (UTC)

Something I wish could be noted in the article.

Advisory speed limit signs -- those yellow signs often posted where there is a curve in the road -- are often placed there not just to warn drivers of the curve but because there is limited "sign distance," like when the curved roadway is cut through the vertical face of a hill or mountain and you can't see more than 20 feet ahead of you. I know this is outside the subject matter of the article but in California at least, and maybe other states too, police won't ticket you for violating an advisory speed limit sign unless some kind of incident, or accident, occurs. This is just a sore spot of mine cause there is a winding road I travel a lot through with a bunch of blind curves and drivers pay no attention to the advisory speed limits. As dangerous at that road is for cars I've seen bicyclists in packs on that road, and speeding around a blind curve there they are. 2600:8801:BE26:2700:41C1:9981:7CD6:CC06 (talk) 18:38, 14 September 2021 (UTC) James.