11

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

Be able to define and use the stopping sight Be able to define and use the stopping sight distance equationdistance equationBe able to explain the difference between stopping Be able to explain the difference between stopping sight distance and decision sight distancesight distance and decision sight distanceBe able to tell the components of passing sight Be able to tell the components of passing sight distancedistanceBe able to explain the difference between passing Be able to explain the difference between passing sight distance defined by AASHTO and passing sight distance defined by AASHTO and passing sight distance defined by MUTCD that is used to sight distance defined by MUTCD that is used to locate no passing zoneslocate no passing zonesKnow how to measure these sight distancesKnow how to measure these sight distances

Objectives:

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3.2 Sight Distance3.2 Sight Distance

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

3.2.1 General Considerations:

33

Four Aspects of Sight Distance Four Aspects of Sight Distance discussed in this section (3.2.1) discussed in this section (3.2.1) Sight distances needed for stopping - Sight distances needed for stopping - applicable on all highwaysapplicable on all highwaysSight distances needed for the passing of Sight distances needed for the passing of overtaken vehicles – applicable only to two-overtaken vehicles – applicable only to two-lane highwayslane highwaysSight distances needed for decisions at Sight distances needed for decisions at complex locationscomplex locationsCriteria for measuring the above sight Criteria for measuring the above sight distances for use in designdistances for use in design

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

44

3.2.2 Stopping Sight Distance (p.3-2)3.2.2 Stopping Sight Distance (p.3-2)Stopping sight distance = sufficiently long sight Stopping sight distance = sufficiently long sight distance for a vehicle traveling at or near the design distance for a vehicle traveling at or near the design speed to stop before reaching a stationary objectspeed to stop before reaching a stationary object

G

gg

vvtd

2

2

t = 2.5 sec (can be 1.0 to 1.5 sec on urban streets)

α = 11.2 ft/s2

With this α, coefficient of friction is 0.35.

(90th percentile values)AASHTO Greenbook 1994 had this figure – longitudinal friction factor vs. speed. This was replaced by α/g beginning the 2001 edition of the Greenbook.

(3-2)

55

Deriving the braking distance formulaDeriving the braking distance formula

66

Deriving the braking distance formula Deriving the braking distance formula (continued)(continued)

Forces acting on this free body is at equilibrium:

W*sinr -W*f*cosr = W*a/ga 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 = atNow t = u/a. Plug in this in the RHS of x

x = ½a t2 = ½a(u2/a2)

x = (½)(u2/a)

Solve for a: a = -u2

2x

Now, we get:

a = u2

2xIt’s deceleration, so add -.

77

Deriving the braking distance formula Deriving the braking distance formula (continued)(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 * cosrOnce you know this, you can easily follow the derivation in the text. Go to page 64.

Db = u2

2g(f - G)Eq. 3.18

Stopping Sight Distance = ut + u2

2g(f ± G)

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Table 3-1 and 3-2 SSD ValuesTable 3-1 and 3-2 SSD Values

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.

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3.2.3 Decision Sight Distance3.2.3 Decision Sight Distance

DSDs are a bit longer than SSDs to allow the driver to DSDs are a bit longer than SSDs to allow the driver to take unexpected actions. Usually PIEV is longer than take unexpected actions. Usually PIEV is longer than regular cases for SSDsregular cases for SSDs. DSDs are used:. DSDs are used:– At locations of complex decision makingAt locations of complex decision making– Where information is difficult to perceiveWhere information is difficult to perceive– Where unexpected or unusual maneuvers are requiredWhere unexpected or unusual maneuvers are requiredLike interchanges, merge areas, so many signs clustered at one Like interchanges, merge areas, so many signs clustered at one

point, etc. (with a lot of visual “noise”)point, etc. (with a lot of visual “noise”)

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

Assumptions for computation and measurement:

3.5 ft eye height and 2 ft object height.

1010

Table 3-3. DSDsTable 3-3. DSDs

For A and B

For C, D, and E

(Different formulas are used.)

1111

3.2.4 Passing Sight Distance3.2.4 Passing Sight DistanceCriteria for Design

1. The overtaken vehicle travels at uniform speed.

2. The passing vehicle has reduced speed and trails the overtaken vehicle as it enters a passing section.

3. 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.

4. 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.

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

1212

How the min. passing sight distance for 2-How the min. passing sight distance for 2-lane highways is determined.lane highways is determined.

• 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.

1313

Elements of PSDElements of PSD

d3 = Clearance length, empirical

d4 = Distance traversed by an opposing vehicle =

d1 = Initial maneuver distance

d2 = Distance while passing vehicle occupies left lane

2/3 of d2

Note: m value changed from 10 mph (GB2004) to 12 mph (GB2011).

1414

Exhibit 3-5 Elements of PSDExhibit 3-5 Elements of PSD

This exhibit 3-5 was deleted in GB2011.

1515

PSD Design Values (GB2004)PSD Design Values (GB2004)

Exhibit 3-6

Exhibit 3-7These exhibits were omitted from GB2011.

PSD Design Values (GB2011), p.3-9PSD Design Values (GB2011), p.3-9

1616These values are those of PSD from MUTCD.

Comparison between SSD & PSDComparison between SSD & PSD

1717

1818

MUTCD on Passing Sight Distance (p.3-12)MUTCD on Passing Sight Distance (p.3-12)

MUTCD 2009

Table 3-5

GB2010: MPZ Lengths to be included GB2010: MPZ Lengths to be included in Traffic Operational Analyses, p.3-14in Traffic Operational Analyses, p.3-14

1919

2020

MUTCD on Passing Sight Distance MUTCD on Passing Sight Distance (cont)(cont)

MUTCD 2009

2121

3.2.6 Criteria for Measuring Sight Distance3.2.6 Criteria for Measuring Sight Distance

Height of driver’s eyeHeight of driver’s eye– 3.5 ft from the pavement surface3.5 ft from the pavement surface

Height of objectHeight of object– For SSD = 2.0 ftFor SSD = 2.0 ft– For PSD = 3.5 ftFor PSD = 3.5 ft

Sight obstructionsSight obstructions– On tangents On tangents caused by crest vertical curves caused by crest vertical curves– On horizontal curves On horizontal curves the road surface on a crest the road surface on a crest

curve, or something outside the traveled way, such as curve, or something outside the traveled way, such as a longitudinal barrier, the back slope of a curve a longitudinal barrier, the back slope of a curve section, a tree, etc.section, a tree, etc.

2.75 ft for SSD

3.5 ft for PSD

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Measuring and Recording SD on PlansMeasuring and Recording SD on Plans

Check sight distance in Check sight distance in the preliminary design the preliminary design stages (graphical stages (graphical evaluation)evaluation)After the horizontal and After the horizontal and vertical alignments are vertical alignments are tentatively established tentatively established Examine SD by direct Examine SD by direct scaling on the plansscaling on the plansMeasure w.r.t. the Measure w.r.t. the centerline, but centerline, but preferably: preferably: – SSD: between the SSD: between the

points on the one traffic points on the one traffic lanelane

– PSD: from the middle of PSD: from the middle of one lane to the middle one lane to the middle of the other laneof the other lane

Figure 3-2