Sight distance is simply defined as “the longest distance a driver can see in front of him.”

Sight distance may also be perceived as the length of carriageway visible to a driver in both horizontal and vertical planes.

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

Obstructions to the driver’s view may arise through various objects such as parked vehicles, plants on the inside of curves, cut sections, buildings, etc.

For safe driving, certain minimum sight distances should be prescribed.

Sight distances that are commonly provided at the design stage include:

1. Stopping sight distance2. Passing sight distance3. Intersection sight distance4. Sight distance on horizontal curves5. Sight distance on vertical curves

4.1 STOPPING SIGHT DISTANCE

The clear distance ahead needed by a driver to bring his vehicle to a stop before meeting a stationary or slow-moving object on his way is known as the safe stopping sight distance.

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

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

Perception time is the time which elapses between the instant the driver sees the hazard and the realization that brake action is required.

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Sight Distance4

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

Perception-reaction time = Perception time + Reaction time

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

For safe and comfortable design, a combined time of 2s is suggested. For design purposes perception-reaction time of 1.5 s is assumed for urban areas while 2.5 s is assumed for rural areas.

Perception-reaction distance is the distance traveled during the perception-reaction time.

Perception-reaction distance = 0.278tV

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

Braking distance is the distance needed by a vehicle to decelerate to a stop on a level road after the brakes have been applied.

Braking distance,

where;V = initial speed (km/hr)f = longitudinal coefficient of friction (developed between the tyre and the roadsurface)

The longitudinal coefficient of friction proposed for certain design speeds are as follows:

Design speed, 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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Eye and object heights used should ensure that there is an envelope of clear visibility which enables drivers of low cars to see low objects on the carriageway, and drivers of high vehicles to see portions of other vehicles, even though bridge soffits at sag curves and overhanging tree branches may be in the way. Eye heights are generally between 1.05 m – 2.00 m, while object heights are between 0.26 m – 2.00 m.

Generally,

Stopping sight distance = Perception-reaction distance + Braking distance

On flat roads,

Stopping sight distance,

On slopes,

Stopping sight distance,

Where; n = gradient (%)

EXAMPLE 1

A motorist, traveling at 60 km/hr on a steep rural road with a gradient of 8%, sees an obstruction on the carriageway ahead of him. Calculate the minimum stopping sight distance required.

(Please attend class for the solution)

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4.2 PASSING SIGHT DISTANCE

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

Figure 4-1 shows the four components of the minimum distance required for safe passing on two-way, two-lane roads.

Figure 4-1: Components of minimum safe passing sight distance

d1 = perception-reaction distance traveled by a vehicle while its driver decides if it is safe to pass the vehicle in front = vs t1

where; vs = speed of the slower vehicle (m/s) and t1 = time taken for the driver to decide on making the pass (s),

usually 3.5 s

d2 = the overtaking distance traveled by the overtaking vehicle in carrying out the actual passing maneuver

= 2s + vs

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where;s = safe clearance distance between the fast and slow vehicles= 0.7vs + 6vs = speed of the slower vehicle (m/s)a = acceleration (m/s2)

d3 = the safe distance between the overtaking vehicle and the opposing vehicle at the instant the overtaking vehicle returns to its correct lane = vo t3

where; vo = speed of the oncoming vehicle (m/s) and t3 = safety time (s), usually 1.5 s

d4 = the closing distance traveled by the opposing vehicle during the passing maneuver (this distance is sometimes taken as 2/3 d2)

Thus, the safe passing sight distance, PSD = d1 + d2 + d3 + d4

Table 4-1 shows passing sight distance values according to JKR.

Table 4-1: Passing Sight Distance (JKR)

Design Speed (km/h) Passing Sight Distance (m)120100806050403020

800700550450350250200200

EXAMPLE 2

A vehicle traveling at 80 km/hr wants to overtake a slower vehicle in front. The speed of the oncoming vehicle is 70 km/hr. Calculate the minimum passing sight distance required for this maneuver. Assume the acceleration, a is 1.0 m/s2, and the speed difference between the faster vehicle and the slower vehicle is 16 km/h.

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(Please attend class for the solution)

4.3 INTERSECTION SIGHT DISTANCE

The operator of a vehicle approaching an intersection at grade should have an unobstructed view of the whole intersection and a length of the intersecting road sufficient to permit control of the vehicle to avoid collision.

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

(a) there must be a sufficient unobstructed view to recognize 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 the stop line

In order that drivers will see the appropriate traffic there should be an area of sight unobstructed by buildings or other objects across the corners of an intersection. This is known as the sight triangle as shown in Figure 4-2:

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Figure 4-2: Intersection sight distance

AB = stopping sight distance based on operating speed on road XBC = stopping sight distance based on operating speed on road YAC = sight line

Any object within the sight triangle high enough above the elevation of the adjacent roadways to constitute a sight obstruction should be removed or lowered. Such objects include cut slopes, trees, bushes and other erected objects. Parking within the sight triangle should also be eliminated.

4.3.1 SIGHT DISTANCE FOR APPROACH

(1) Signalized IntersectionsSight distance for approach,

Where;t = total reaction time (urban = 6 s, rural = 10 s)a = acceleration (maximum allowable acceleration = 1.96 m/s2)V = vehicle speed or design speed (in km/hr)

(2) Priority Intersections

Use the similar equation as for sight distance of approach at signalized intersections. However, total reaction time is taken as 2

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sec because decision making is not required as every driver must stop.

4.3.2 SIGHT DISTANCE FOR DEPARTURE

Sight distance for departure,

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

J-value for rural areas is 2 s, while for urban and suburban areas is 1.0 s to 1.5 s.

ta values can be obtained from Figure 4-4. It depends on the distance S which the crossing vehicle must travel to cross the major road.

Figure 4.3: Sight distance for departure

Distance traveled during crossing maneuver, S = D + W + L

Where;

D = distance from near edge of pavement of front of stopped vehicle (for design purposes, taken as 3 m)W = width of pavement along path of crossing vehicle (in m)

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SD

S

D

WL

AB

Sight Line

Va

L = overall length of vehicle (5 m for passenger cars, 10 m for single unit trucks and 15 m for semi-trailers)

Figure 4-4: Time required to accelerate and traverse the distance S (ta)

Table 4-2 gives the stopping sight distance for intersection approach at Signalized Intersections and Stop-Controlled Intersections, as recommended by JKR.

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Table 4-2: Sight Distance for Intersection Approach (JKR)

Design Speed Of Major

Road (km/h)

Signal Control Stop Control(on Minor

Road)*Rural Urban100806050403020

48035024019014010060

3702601701301007040

26017010580553520

* On Major Roads of Stop Controlled Intersections, the Stopping Sight Distances must comply to those given in Table 4.3

EXAMPLE 3

A car is traveling at 75 km/hr along a secondary road approaching an intersection with priority control. The car departs from the intersection at a speed of 60 km/h. The width of pavement along the path where the vehicle crosses is 7.0 m. Calculate the required sight distance for approach and departure.

(Please attend class for the solution)

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4.4 SIGHT DISTANCE ON HORIZONTAL CURVES

Difficulties in providing the required safe stopping and passing sight distances are most commonly encountered in urban road design where the alignment constraints are such that the desired visibility can only be achieved at considerable 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 hinder visibility.

In both urban and rural areas, safety fences in the central reservation between dual carriageways can hinder the achievement of the minimum stopping distance in the inside lane because of the low object height.

Figure 4-5 illustrates the situation where the required sight distance lies wholly within the length of the curve, L is assumed equal to the required sight distance, S. M is the minimum offset clearance desired between the centerline and any lateral obstruction.

Therefore when S < L: , where R = horizontal curve radius

Figure 4-6 illustrates the situation where S is greater than the available length of curve, L and overlaps onto the tangents for a distance of l on either side.

Therefore when S > L:

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Figure 4-5: Sight distance for horizontal curve (S L)

Figure 4-6: Sight distance on horizontal curve (S > L)

EXAMPLE 4

The figure below illustrates the proposed site for the construction of a building that is adjacent to a horizontal curve section of a rural highway. The suggested offset clearance is 10 m. The highway design speed is 100 km/hr, while the curve length and curve radius is 200 m and 600 m respectively. Drivers’ perception-reaction time is taken as 2.5 seconds and the coefficient of friction between the

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Straight line

Straight line

tyres and the road surface is 0.28. Is the suggested offset clearance adequate to allow for safe stopping sight distance?

(Please attend class for the solution)

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4.4.1 STOPPING SIGHT DISTANCE ON HORIZONTAL CURVES

The following shows how stopping sight distance (SSD) on a horizontal curve can be calculated given the curve radius (R) and middle ordinate (M).

Figure 4-7: Illustration of SSD on horizontal curveBased on the diagram in Figure 4-7,

where L = length of curve, R = curve radius

where Rv = radius to the travel path of the vehicle

where M = middle ordinate

where Ms = middle ordinate necessary to provide SSD

By substituting into we obtain the following:

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Tables 4-3 gives SSD values according to local standards, while Table 4-4 gives SSD values according to AASHTO standards.

Table 4-3: Stopping Sight Distance (JKR and LLM)

Highway Agency

Design Speed (km/h)

Stopping Sight Distance (m)

Malaysian Highway Authority (LLM)

14012010080

325225150100

Public Works Department (JKR)

120100806050403020

2852051408565453020

Table 4-4: Stopping Sight Distance (AASHTO)

Design Speed (km/h)

Stopping Sight Distance (m)AASHTO 2000 AASHTO 1994

Design Desirable Minimum30405060708090100

35506585105130160185

29.644.462.884.6

110.8139.4168.7205.0

29.644.457.474.394.1

112.8131.2157.0

EXAMPLE 5

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A horizontal curve on a U5 highway is designed with a 700 m radius, 3.6 m lanes, and a 100km/hr design speed. Determine the distance that must be cleared from the inside edge lane to provide sufficient sight distance for desirable and minimum SSD.

(Please attend class for the solution)

4.5 SIGHT DISTANCE ON VERTICAL CURVES

A vertical curve provides a smooth transition between successive tangent gradients in the road profile.

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

The discomfort experienced is minimized by restricting the gradients and by using a type and length of vertical curve which allows the radial force to be experienced gradually and uniformly. Sight distance requirements are also aided by the use of long vertical curves on both crest and sag curves.

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For Crest Curves:

Figure 4-8: Sight distance on crest vertical curve (S L)

Figure 4-8 illustrates the condition where the required sight distance S is contained within the available length of the vertical curve L.

When S < L:

where; A = difference in grades h1 = eye heighth2 = object height

Figure 4-9: Sight distance on crest vertical curve (S > L)

Figure 4-9 illustrates the condition where S is greater than L and overlaps on either sides of the vertical curves.

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When S > L:

For Sag Curves:

When S < L:

When S > L:

Where;D = vertical clearance (ideally taken as 5.7 m)

EXAMPLE 6

A car is traveling at 90 km/hr on a crest vertical curve connecting grades of +1% and -2% and having a curve length of 300 m. Further ahead of the car, a box from a truck has fallen onto the travel lane. The height of the box is 500 cm. Eye height is taken as 1.06 m. Ignore the effects of grades on stopping sight distance. The road is in a rural area. Calculate the minimum length required for the car to stop safely and avoid colliding with the box.

(Please attend class for the solution)

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