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Chapter 3: Elements of Design Sight Distances (p.3-1 to 3-18 ). Be able to define and use the stopping sight distance equation Be able to explain the difference between stopping sight distance and decision sight distance Be able to tell the components of passing sight distance
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3.2.1 General Considerations:
Sight distance is the length of the roadway ahead that is visible to the driver.
t = 2.5 sec (can be 1.0 to 1.5 sec on urban streets)
a = 11.2 ft/s2
AASHTO Greenbook 1994 had this figure – longitudinal friction factor vs. speed. This was replaced by a/g beginning the 2001 edition of the Greenbook.
(90th percentile values)
a = -
2xDeriving the braking distance formula (continued)
Forces acting on this free body is at equilibrium:
W*sinr -W*f*cosr = W*a/g
a is unknown. We want to use the known values (initial speed u, and distance x) to determine a. We assume first the vehicle accelerated from speed 0 to u.
x = ½at2 & u = at
Now t = u/a. Plug in this in the RHS of x
x = ½a t2 = ½a(u2/a2)
Now, we get:
x = (½)(u2/a)
It’s deceleration, so add -.
Solve for a:
2g(f - G)
Stopping Sight Distance = ut +
2g(f ± G)
Deriving the braking distance formula (continued)
The braking distance is a horizontal distance (do you know why we use a horizontal distance?) while x is the distance along the slope; therefore,
Db = x * cosr
Once you know this, you can easily follow the derivation in the text. Go to page 64.
SSDs shown here are for passenger cars. Truck drivers’ eye heights are much higher and they can see further. Hence, SSDs for just trucks are not used.
Like interchanges, merge areas, so many signs clustered at one point, etc. (with a lot of visual “noise”)
Assumptions for computation and measurement:
3.5 ft eye height and 2 ft object height.
For C, D, and E
For A and B
(Different formulas are used.)
Criteria for Design
d1 = Initial maneuver distance
d3 = Clearance length, empirical
d4 = Distance traversed by an opposing vehicle =
2/3 of d2
d2 = Distance while passing vehicle occupies left lane
Note: m value changed from 10 mph (GB2004) to 12 mph (GB2011).
This exhibit 3-5 was deleted in GB2011.
These exhibits were omitted from GB2011.
These values are those of PSD from MUTCD.
2.75 ft for SSD
3.5 ft for PSD