If we have a highway design, so how to calculate the design speed?
Dear Ibrahim, there exist mathematical models of traffic flow; see here for the start: https://en.wikipedia.org/wiki/Traffic_flow
So depending on the car density and speed, there might be traffic waves and congestion. What do you mean by design speed of a highway? If you can predict density of cars, you can solve this mathematical problem of what real speed would be for any desired speed. All this depends also on time of drivers' reaction, secure interval, that can differ across countries.
Clearly, police can set a sign with speed limit if there are some danger caused by highway geometry. If you expected different answer, please formulate your question more exactly.
Thanks dear Yuri Yegorov, the question is when we design a new speed we depend on a certain design speed which then calculates other geometric parameters (i.e curves). Now the question is how to reverse engineering the calculation of that design speed selected during the design stage.
Ibrahim, if you get analytical function between curvature and speed, you can simply take inverse function. If it is discrete mapping, then you draw graph and interpolate. But I think that geometry of landscape defines this curvature, and this gives the highest secure speed. It may happen that cars simply do not need it (let say, secure speed in 180 km/h, while only few choose to drive above 150 km/h, if the state laws allow) - then it is not binding. But I think that you are more interested in the case when desired speed is 120 km/h, but the landscape allows for curvatures with only 100 km/h, and for less curvature you need high investment in artificial landscape. Then it is also economic problem.
hi Ibrahim, since you're in Malaysia, I supposed you're asking about the design speed of the highway in Malaysia. I came across "A Guide On Geometric Design Of Road" (as attached). Hope it helps.
Dear Maher
its so easy ..by conducting spot speed study for the section..and then tabulate the data and draw a relationship between the percent accumulative frequency and speed..then you can consider the 98 percent as a design speed..regards
I think is percentile. Also, in existing highway you may have access to all geometry elements (curve radii, tangent lenghts, etc.), so you can calculate several design speeds for all those elements using reserse equations, and finally get the mean speed and that would be the design speed of the highway project. More information in https://www.witpress.com/books/978-1-84564-156-6
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