TOPIC 2- Sight Distance

8/12/2019 TOPIC 2- Sight Distance

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ROAD GEOMETRICDESIGN Chapter 2

SIGHT DISTANCE

-Topic 2-


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SIGHT DISTANCE:

INTRODUCTION TO SIGHT DISTANCESight Distance = The longest distance a driver can see infront of him

Sight distance may also be perceived as the length ofcarriageway visible to a driver in both horizontal and verticalplanes.

Sight distance is the most important feature in the safe andefficient operation of a highway.

Obstructions to the drivers view may arise through variousobjects such as parked vehicles, plants on the inside ofcurves, cut sections, buildings, etc.

For safe driving, certain minimum sight distances should be

prescribed.


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INTRODUCTION TO SIGHT DISTANCESight distances that are commonly provided at the designstage include:

1. Stopping sight distance2. Passing sight distance3. Intersection sight distance4. Sight distance on horizontal curves

5. Sight distance on vertical curves


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)The clear distance ahead needed by a driver to bring hisvehicle to a stop before meeting a stationary or slow-movingobject on his way is known as the safe stopping sightdistance.

The calculation of the minimum distance required to stop avehicle before it hits a stationary or slow-moving objectinvolves establishing values for speed, perception-reactiontime, braking distance and eye and object heights.

The vehicle speed used in safe stopping sight distancecalculations is normally the design speed.


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)Perception time is the time which elapses between theinstant the driver sees the hazard and the realization thatbrake action is required.

Reaction time is the time taken by the driver to actuate thebrake pedal, after realizing the need to brake, until thebrakes start to take effect.

Perception-reaction time = perception time + reaction time


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)

Field measurements indicate that combined perception-reaction time typically vary form 0.5 s in difficult terrainwhere drivers are more alert, to 1.5 s under normal road

conditions.For safe and comfortable design, a combined time of 2s issuggested.

For design purposes perception-reaction time of 1.5 s isassumed for urban areas while 2.5 s is assumed for ruralareas.


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)Perception-reaction distance is the distance traveled duringthe perception-reaction time.

Perception-reaction distance = 0.278 tV

where; t = perception-reaction time (in seconds) V = initial speed (in km/hour)


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)Braking distance is the distance needed by a vehicle todecelerate to a stop on a level road after the brakes havebeen applied.

Braking distance,

where; V = initial speed (km/hr) f = longitudinal coefficient of friction (developed

between the tyre and the road surface)

f V d

254

2


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)The longitudinal coefficient of friction proposed for certaindesign speeds are as follows:

Designspeed, V (km/hr)

30 40 50 60 70 80 90 100 110 120

Coefficient

of friction,f

0.40 0.38 0.35 0.33 0.31 0.30 0.30 0.29 0.28 0.28


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)The f -value depends on the following factors:

- carriageway conditions wet roads are normallyassumed as water acts as a lubricant between thecarriageway and tyres.

- tyre quality a well-patterned tread provides goodescape channels for bulk water and a radial plyincreases the tyre-road contact area.

- speed the higher the vehicle speed the less is thecontact time available to expel water from betweenthe tyre and the carriageway.


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)- carriageway macrotexture and microtexture a rough

macrotexture helps with the removal of bulk water and themaintenance of skidding resistance at high speeds, while theharsh microtextures of surfacing materials add to skidresistance as the puncture and disperse the thin film ofwater remaining after the removal of the bulk water by thetyre tread and the carriageway surface.


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)Eye and object heights used should ensure that there is anenvelope of clear visibility which enables drivers of low cars tosee low objects on the carriageway, and drivers of high vehiclesto see portions of other vehicles, even though bridge soffits atsag curves and overhanging tree branches may be in the way.

Eye heights are generally between 1.05 m 2.00 m, whileobject heights are between 0.26 m 2.00 m.

Generally,

Stopping sight distance = perception-reaction distance + braking distance


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LESSON 1: STOPPING SIGHT DISTANCE (SSD)On flat roads,

Stopping sight distance,

On slopes,

Stopping sight distance,

Where; n = gradient (%)

f

V tV SSD

254278.0

2

100254

278.02

n f

V tV SSD


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Example 1 (pg 44; problems 2-1)

A driver takes 3.5 sec to react to acomplex situation while travelling at aspeed of 60 km/hr. How far does the

vehicle travel before the driver initiatesa physical response to the situation (i.e.putting his/her foot on the brake)?

LESSON 1: STOPPING SIGHT DISTANCE (SSD)


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Example 2 (pg 44; problems 2-6)What is the safe stopping distance for a

section of rural freeway with a designspeed of 80 km/hr on a 3%downgrade?


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Example 3 (pg 40; paragraph 3)

An accident investigator estimates that a vehicle hit abridge abutment at a speed of 32.19 km/hr, basedon his/her assessment of damage. Leading up to theaccident location, he/she observes skid marks of30.48 m on the pavement (f = 0.35) and 22.86 m onthe grass shoulder (f = 0.25). There is no grade. Anestimation of the speed of the vehicle at the

beginning of the skid marks is desired.


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LESSON 2: PASSING SIGHT DISTANCE (PSD)

Sufficient sight distances must be available on two-way,two-lane roads to enable faster vehicles to safely overtakeslower ones, without causing disruption to traffic flow on the

opposite direction.

Figure 2-1 shows the four components of the minimumdistance required for safe passing on two-way, two-laneroads.
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d 1 = perception-reaction distance traveled by a vehiclewhile its driver decides if it is safe to pass the vehicle infront.

d 1 = v s t 1

where;v s = speed of the slower vehicle (m/s) and t 1 = time taken for the driver to decide on

making the pass (s), usually 3.5 s


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d 2 = the overtaking distance traveled by the overtakingvehicle in carrying out the actual passing maneuver

d 2 = 2 s + v s

where; s = safe clearance distance between the fast and

slow vehicles= 0.7 vs + 6

v s = speed of the slower vehicle (m/s) a = acceleration (m/s 2 )

a s4


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d 3 = the safe distance between the overtaking vehicle andthe opposing vehicle at the instant the overtakingvehicle returns to its correct lane

d 3 = v o t 3

where;

v o = speed of the oncoming vehicle (m/s) andt 3 = safety time (s), usually 1.5 s


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d 4 = the closing distance traveled by the opposing vehicleduring the passing maneuver (this distance issometimes taken as 2/3 d 2 )

Thus, the safe passing sight distance,

PSD = d1 + d2 + d3 + d4

* Note: It is always assumed that the speed difference betweenthe faster vehicle and the slower vehicle is 16 km/hr


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Table 2-1 shows PSD values according to JKR:

Design Speed (km/hr) Passing Sight Distance (m)

120100806050403020

800700550450350250200200


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Example 2:

A vehicle traveling at 80 km/hr wants to overtakea slower vehicle in front. The speed of theoncoming vehicle is 70 km/hr. Calculate theminimum PSD required for this maneuvre.

Assume the acceleration, a is 1.0 m/s 2, and thespeed difference between the faster vehicle andthe slower vehicle is 16 km/hr.


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- Intersection sight distance is needed to permit control of

the vehicle to avoid collision.

- For the sight of distance of the driver of a vehicle passingthrough an intersection, two aspects must be considered:

(a) there must be a sufficient unobstructed view torecognize the traffic signs or traffic signals at the

intersection(b) there must also be a sufficient sight distance to make

a safe departure after the vehicle has stopped at thestop line

LESSON 3: INTERSECTION SIGHT DISTANCE


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To get the view of appropriate traffic - The area of sightshould be unobstructed by buildings or other objects acrossthe corners of an intersection. This is known as the sighttriangle ( see Figure 2-2 ).

Any object within the sight triangle high enough above theelevation of the adjacent roadways to constitute a sightobstruction should be removed or lowered.

Such objects include cut slopes, trees, bushes and othererected objects also should be removed.

Parking within the sight triangle should also be eliminated.

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(A) SIGHT DISTANCE APPROACH

(1) SIGNALIZED INTERSECTIONSSight distance for approach, Where;

Sa = Sight distance traveled from the driver recognize trafficsignals to vehicle stop with applying brake

t = total reaction time (urban = 6 s, rural = 10 s)a = acceleration (maximum allowable acceleration = 1.96

m/s 2)

V = vehicle speed or design speed (in km/hr)

2

6.321

6.3

V

aVt S A

LESSON 3: INTERSECTION SIGHTDISTANCE:


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(2) PRIORITY INTERSECTIONS

Equation = similar as for sight distance of approach atsignalized intersections.However, total reaction time is taken as 2 s .

Reason decision making is not required as every drivermust stop.



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(B) SIGHT DISTANCE FOR DEPARTURE

Sight distance for departure,

)(278.0 a D t J V S Where;

SD = min sight distance along major road fromintersection (m) V = vehicle speed or design speed (in km/h)J = sum of perception time and the time required to

shift to first gear or actuate an automatic shift (inseconds)

ta = time required to accelerate and traverse thedistance S to clear the major road (in seconds)

*Note : J-value for rural areas is 2 s, while for urban andsuburban areas is 1.0 s to 1.5 s



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ta values can be obtained from Figure 2-3 . It depends on thedistance S w hich the crossi ng vehicle must travel to cross themajor road. ( see Figure 2-4 )Distance traveled during crossing maneuver,

S = D + W + LWhere;

S = distance vehicle must travel to cross major roadD = distance from near edge of pavement of front

of stopped vehicle (for design purposes, taken as 3m)W = width of pavement along path of crossing vehicle

(in m)L = overall length of vehicle (5 m for passenger cars,

10 m for single unit trucks and 15 m for semi-trailers)

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Table 2-2 gives the stopping sight distance for intersection approachat Signalized Intersections and Stop-Controlled Intersections, asrecommended by JKR:

Major Speed ofMajor Road

(km/hr)

Signal Control Stop Control(on Minor

Road)*Rural Urban

1008060504030

20

480350240190140100

60

37026017013010070

40

260170105805535

20


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*On Major Roads of Stop Controlled Intersections, the Stopping SightDistances must comply to those given in Table 4-3 (page 61).

Example 3:

A car traveling at 75 km/hr along a secondary roadapproaching an intersection with priority control. The cardeparts from the intersection at a speed of 60 km/hr. Thewidth of pavement along the path where the vehicle crossesis 7.0 m. Calculate the required sight distance for approachand departure.


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LESSON 4: SIGHT DISTANCE ON

HORIZONTAL CURVES

Difficulties in providing the required safe stopping andpassing sight distances are most commonly encountered inurban road design where the alignment constraints are suchthat the desired visibility can only be achieved atconsiderable financial and environmental costs.

In rural areas, diverse obstructions at the side of the road,e.g. buildings, bridge supports, slopes of cuttings, solid

fences, or uncut grass on or adjacent to verges, can hindervisibility.


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LESSON 4: SIGHT DISTANCE ON

HORIZONTAL CURVES

In both urban and rural areas, safety fences in the centralreservation between dual carriageways can hinder theachievement of the minimum stopping distance in the insidelane because of the low object height.


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LESSON 4: SIGHT DISTANCE ONHORIZONTAL CURVES

Figure 2-4: Sight distance for horizontal

curve (S L)

Figure 2-4 illustratesthe situation wherethe required sightdistance lies whollywithin the length ofthe curve, L isassumed equal tothe required sight

distance, S . M is theminimum offsetclearance desired between thecenterline and any

lateral obstruction.


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Therefore when S < L : ;where R = horizontal curveradius R

S M

8

2

Figure 2-5: Sightdistance on horizontal

curve (S > L)

Figure 2-5 illustratesthe situation whereS is greater than theavailable length ofcurve , L and overlapsonto the tangentsfor a distance of l oneither side.


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Therefore when S > L :

Where;M = desired minimum clearance offsetL = length of curve

R = horizontal curve radiusS = required sight distance

R LS L

M 8

)2(


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Example 4:

The figure below illustrates the proposed site for theconstruction of a building that is adjacent to a horizontalcurve section of a rural highway. The suggested offsetclearance is 10 m. The highway design speed is 100 km/hr,while the curve length and curve radius is 200 m and 600 mrespectively. Drivers perception -reaction time is taken as2.5 s and the coefficient of friction between the tyres andthe road surface is 0.28. Is the suggested offset clearanceadequate to allow for safe stopping sight distance?


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LESSON 5: STOPPING SIGHT DISTANCEON HORIZONTAL CURVES

The followingshows howstopping sightdistance (SSD)on a horizontalcurve can be

calculated giventhe curve radius(R) and middleordinate (M).


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Based on the diagram:

R L

180 length of sight line, l = 2R vsin( s /2)

v s R SSD

180 v s R

SSD

180

)2

cos1(

R M )2

cos1( s v s R M


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v s R

SSD

180 )2

cos1( s v s R M

2

180

cos1 v v s R

SSD

R M

Substitute into

v

v s R

SSD R M

90cos1

v

svv

R M R R

SSD 1cos90


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Highway Agency Design Speed (km/h) Stopping Sight Distance (m)

Malaysian Highway

Authority (LLM)

140

12010080

325

225150100

Public WorksDepartment (JKR)

12010080

6050403020

285205140

8565452020

Table 4-3: SSD (JKR & LLM)


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Design speed (km/h) Stopping Sight Distance (m)

AASHTO 2001 AASHTO 1994

Design Desirable Minimum

30405060708090

100

35506585

105130160185

29.644.462.884.6

110.8139.4168.7205.0

29.644.457.474.394.1

112.8131.2157.0

Table 4-4: SSD (AASHTO)


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Example 5:

A horizontal curve on a U5 highway isdesigned with a 700 m radius, 3.6 m lanesand a 100 km/hr design speed. Determinethe distance that must be cleared from theinside edge lane to provide sufficient sightdistance for desirable and minimum SSD.


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LESSON 6: SIGHT DISTANCE ON VERTICAL

CURVES A vertical curve provides a smooth transition betweensuccessive tangent gradients in the road profile.

As a motorist traverses a vertical curve, a radial force acts onthe vehicle and tries to force it away from the centre of thecurvature and this may give the motorist some discomfort.

The discomfort experienced is minimized by restricting thegradients and by using a type and length of vertical curvewhich allows the radial force to be experienced graduallyand uniformly.

Sight distance requirements are also aided by the use of longvertical curves on both crest and sag curves.


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CURVESFor crest curves;

Figure 2-6: Sight distance on crest vertical curve (S L)


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CURVESFigure 2-6 illustrates the condition where the required sightdistance S is contained within the available length of thevertical curve L .

When S < L :

where; A = difference in gradesh 1 = eye heighth 2 = object height

221

2min )22( hh

AS L


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CURVES

Figure 2-7: Sight distance on crest vertical curve (S > L)

Figure 2-7 illustrates the condition where S is greater thanL and overlaps on either sides of the vertical curves.


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LESSON 6: SIGHT DISTANCE ON VERTICALCURVES

When S > L :

For Sag Curves: (a) When S < L :

(b) When S > L:

Ahh

S L2

21min

)(22

2)(8

8 21

2

min hh D

AS L

A

hh D

S L

2)(8

82

21

min

Where;

D = verticalclearance(ideally takenas 5.7 m)


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LESSON 6: SIGHT DISTANCE ON VERTICAL CURVES

Example 6:

A car is traveling at 90 km/hr on a crest verticalcurve connecting grades of +1% and 2% and havinga curve length of 300 m. Further ahead of the car, abox from a truck has fallen onto the travel lane. Theheight of the box is 500 cm. Eye height is taken as1.06 m. Ignore the effects of grades on stoppingsight distance. The road is in a rural area. Calculatethe minimum length required for the car to stopsafely and avoid colliding with the box.


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TOPIC 2- Sight Distance