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Chapter 3: Elements of Design Sight Distances (p.3-1 to 3-18 )

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- 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
- Be able to explain the difference between passing sight distance defined by AASHTO and passing sight distance defined by MUTCD that is used to locate no passing zones
- Know how to measure these sight distances

Objectives:

3.2.1 General Considerations:

- A driver’s ability to see ahead is of the utmost importance in the safe and efficient operation of a vehicle on a highway.
- Diverse types of drivers, from 16 years old to senior drivers (Read a statement in the book about a comparison with train control)
- At minimum, must provide sight distance of sufficient length to safely stop when an unexpected object appears on the traveled way
- On 2-lane 2-way highways, provide sufficient sight distance to enable drivers to occupy the opposing traffic lane for passing overtaken vehicles without hazard
- ON 2-lane 2-way highways, NO PASSING zone must be clearly defined

- Sight distances needed for stopping - applicable on all highways
- Sight distances needed for the passing of overtaken vehicles – applicable only to two-lane highways
- Sight distances needed for decisions at complex locations
- Criteria for measuring the above sight distances for use in design

Sight distance is the length of the roadway ahead that is visible to the driver.

- Stopping sight distance = sufficiently long sight distance for a vehicle traveling at or near the design speed to stop before reaching a stationary object

(3-2)

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)

u2

u2

a = -

a =

2x

2x

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:

u2

Db =

2g(f - G)

u2

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.

Eq. 3.18

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.

- DSDs are a bit longer than SSDs to allow the driver to take unexpected actions. Usually PIEV is longer than regular cases for SSDs. DSDs are used:
- At locations of complex decision making
- Where information is difficult to perceive
- Where unexpected or unusual maneuvers are required
Like interchanges, merge areas, so many signs clustered at one point, etc. (with a lot of visual “noise”)

- If DSDs cannot be provided, supplement with traffic control devices (signs, markers, etc.)

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

- The overtaken vehicle travels at uniform speed.
- The passing vehicle has reduced speed and trails the overtaken vehicle as it enters a passing section.
- When the passing section is reached, the passing driver needs a short period of time to perceive the clear passing section and to react to start his or her maneuver.

- When the passing vehicle returns to its lane, there is a suitable clearance length between it and an oncoming vehicle in the other lane.

- Passing is accomplished under what may be termed a delayed start and a hurried return in the face of opposing traffic. The passing vehicle accelerates during the maneuver, and its average speed during the occupancy of the left lane is 10 mph higher than that of the overtaken vehicle.

- d1 – Distance traversed during perception and reaction time AND during the initial acceleration to the point of encroachment on the left lane.
- d2 – Distance traveled while the passing vehicle occupies the left lane.
- d3 – Distance between the passing vehicle at the end of its maneuver and the opposing vehicle
- d4 – Distance traversed by an opposing vehicle for two-thirds of the time the passing vehicle occupies the left lane, or 2/3 of d2.

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.

Exhibit 3-6

Exhibit 3-7

These exhibits were omitted from GB2011.

These values are those of PSD from MUTCD.

Table 3-5

MUTCD 2009

MUTCD 2009

- Height of driver’s eye
- 3.5 ft from the pavement surface

- Height of object
- For SSD = 2.0 ft
- For PSD = 3.5 ft

- Sight obstructions
- On tangents caused by crest vertical curves
- On horizontal curves the road surface on a crest curve, or something outside the traveled way, such as a longitudinal barrier, the back slope of a curve section, a tree, etc.

2.75 ft for SSD

3.5 ft for PSD

- Check sight distance in the preliminary design stages (graphical evaluation)
- After the horizontal and vertical alignments are tentatively established Examine SD by direct scaling on the plans
- Measure w.r.t. the centerline, but preferably:
- SSD: between the points on the one traffic lane
- PSD: from the middle of one lane to the middle of the other lane

Figure 3-2