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Design speed is the single most important factor that affects the geometric
design.
It directly affects the sight distance, horizontal curve, and the length of
vertical curves.
Since the speed of vehicles vary with driver, terrain etc, a design speed is
adopted for all the geometric design.
Design speed is defined as the highest continuous speed at which individual
vehicles can travel with safety on the highway when weather conditions are
conducive.
Design speed is different from the legal speed limit which is the speed limit
imposed to curb a common tendency of drivers to travel beyond an accepted safe
speed.
Design speed is also different from the desired speed which is the maximum
speed at which a driver would travel when unconstrained by either traffic or
local geometry.
Since there are wide variations in the speed adopted by different drivers, and
by different types of vehicles, design speed should be selected such that it
satisfy nearly all drivers.
At the same time, a higher design speed has cascading effect in other geometric
designs and thereby cost escalation.
Therefore, an 85th percentile design speed is normally adopted.
This speed is defined as that speed which is greater than the speed of 85% of
drivers.
In some countries this is as high as 95 to 98 percentile speed.
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