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DESIGN Chapter 2
SIGHT DISTANCE
-Topic 2-
SIGHT DISTANCE:

INTRODUCTION TO SIGHT DISTANCE
Sight Distance = 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 drivers 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.
INTRODUCTION TO SIGHT DISTANCE
Sight distances that are commonly provided at the design
stage include:

1. Stopping sight distance
2. Passing sight distance
3. Intersection sight distance
4. Sight distance on horizontal curves
5. Sight distance on vertical curves

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

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
LESSON 1: STOPPING SIGHT DISTANCE (SSD)
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.

LESSON 1: STOPPING SIGHT DISTANCE (SSD)
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)

LESSON 1: STOPPING SIGHT DISTANCE (SSD)
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 road surface)
f
V
d
254
2
=
LESSON 1: STOPPING SIGHT DISTANCE (SSD)
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
LESSON 1: STOPPING SIGHT DISTANCE (SSD)
The f-value depends on the following factors:

- carriageway conditions wet roads are normally
assumed as water acts as a lubricant between the
carriageway and tyres.

- tyre quality a well-patterned tread provides good
escape channels for bulk water and a radial ply
increases the tyre-road contact area.

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

LESSON 1: STOPPING SIGHT DISTANCE (SSD)
- carriageway macrotexture and microtexture a rough
macrotexture helps with the removal of bulk water and the
maintenance of skidding resistance at high speeds, while the
harsh microtextures of surfacing materials add to skid
resistance as the puncture and disperse the thin film of
water remaining after the removal of the bulk water by the
tyre tread and the carriageway surface.
LESSON 1: STOPPING SIGHT DISTANCE (SSD)
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

LESSON 1: STOPPING SIGHT DISTANCE (SSD)

Stopping sight distance,

On slopes,

Stopping sight distance,

Where;
n = gradient (%)
f
V
tV SSD
254
278 . 0
2
+ =
|
.
|

\
|

+ =
100
254
278 . 0
2
n
f
V
tV SSD
Example 1 (pg 44; problems 2-1)

A driver takes 3.5 sec to react to a
complex situation while travelling at a
speed of 60 km/hr. How far does the
vehicle travel before the driver initiates
a physical response to the situation (i.e.
putting his/her foot on the brake)?
LESSON 1: STOPPING SIGHT DISTANCE (SSD)
Example 2 (pg 44; problems 2-6)
What is the safe stopping distance for a
section of rural freeway with a design
speed of 80 km/hr on a 3%
Example 3 (pg 40; paragraph 3)
An accident investigator estimates that a vehicle hit a
bridge abutment at a speed of 32.19 km/hr, based
on his/her assessment of damage. Leading up to the
accident location, he/she observes skid marks of
30.48 m on the pavement (f = 0.35) and 22.86 m on
the grass shoulder (f = 0.25). There is no grade. An
estimation of the speed of the vehicle at the
beginning of the skid marks is desired.
LESSON 2: PASSING SIGHT DISTANCE (PSD)
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 2-1 shows the four components of the minimum
distance required for safe passing on two-way, two-lane
LESSON 2: PASSING SIGHT DISTANCE (PSD)
d
1
= perception-reaction distance traveled by a vehicle
while its driver decides if it is safe to pass the vehicle in
front.
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
LESSON 2: PASSING SIGHT DISTANCE (PSD)
d
2
= the overtaking distance traveled by the overtaking
vehicle in carrying out the actual passing maneuver

d
2
= 2s + v
s

where;
s = safe clearance distance between the fast and
slow vehicles
= 0.7vs + 6
v
s
= speed of the slower vehicle (m/s)
a = acceleration (m/s
2
)
a
s 4
LESSON 2: PASSING SIGHT DISTANCE (PSD)
d
3
= the safe distance between the overtaking vehicle and
the opposing vehicle at the instant the overtaking
vehicle returns to its correct lane

d
3
= v
o
t
3

where;
v
o
= speed of the oncoming vehicle (m/s) and
t
3
= safety time (s), usually 1.5 s
LESSON 2: PASSING SIGHT DISTANCE (PSD)
d
4
= the closing distance traveled by the opposing vehicle
during the passing maneuver (this distance is
sometimes 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 between
the faster vehicle and the slower vehicle is 16 km/hr
Table 2-1 shows PSD values according to JKR:

Design Speed (km/hr) Passing Sight Distance (m)
120
100
80
60
50
40
30
20
800
700
550
450
350
250
200
200
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 PSD required for this maneuvre.
Assume the acceleration, a is 1.0 m/s
2
, and the
speed difference between the faster vehicle and
the slower vehicle is 16 km/hr.

- 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 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
LESSON 3: INTERSECTION SIGHT DISTANCE
To get the view of appropriate traffic - The area of sight
should be unobstructed by buildings or other objects across
the corners of an intersection. This is known as the sight
triangle (see Figure 2-2).

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 also should be removed.

Parking within the sight triangle should also be eliminated.
LESSON 3: INTERSECTION SIGHT DISTANCE
(A) SIGHT DISTANCE APPROACH

(1) SIGNALIZED INTERSECTIONS
Sight distance for approach, Where;

Sa = Sight distance traveled from the driver recognize traffic
signals 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 . 3 2
1
6 . 3
|
.
|

\
|
+ =
V
a
Vt
S
A
LESSON 3: INTERSECTION SIGHT
DISTANCE:
(2) PRIORITY INTERSECTIONS
Equation = similar as for sight distance of approach at
signalized intersections.
However, total reaction time is taken as 2 s.
Reason decision making is not required as every driver
must stop.
LESSON 3: INTERSECTION SIGHT
DISTANCE:
(B) SIGHT DISTANCE FOR DEPARTURE
Sight distance for departure,
) ( 278 . 0
a D
t J V S + =
Where;
S
D
= min sight distance along major road from
intersection (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 (in
seconds)
ta = time required to accelerate and traverse the
distance S to clear the major road (in seconds)

*Note : J-value for rural areas is 2 s, while for urban and
suburban areas is 1.0 s to 1.5 s
LESSON 3: INTERSECTION SIGHT
DISTANCE:
ta values can be obtained from Figure 2-3. It depends on the
distance S which the crossing vehicle must travel to cross the
major road. (see Figure 2-4)
Distance traveled during crossing maneuver,

S = D + W + L
Where;
S = distance vehicle must travel to cross major road
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)
L = overall length of vehicle (5 m for passenger cars,
10 m for single unit trucks and 15 m for semi-trailers)
LESSON 3: INTERSECTION SIGHT DISTANCE
Table 2-2 gives the stopping sight distance for intersection approach
at Signalized Intersections and Stop-Controlled Intersections, as
recommended by JKR:
Major Speed of
(km/hr)
Signal Control Stop Control
(on Minor
Rural Urban
100
80
60
50
40
30
20
480
350
240
190
140
100
60
370
260
170
130
100
70
40
260
170
105
80
55
35
20
*On Major Roads of Stop Controlled Intersections, the Stopping Sight
Distances must comply to those given in Table 4-3 (page 61).

Example 3:

A car 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/hr. The
width of pavement along the path where the vehicle crosses
is 7.0 m. Calculate the required sight distance for approach
and departure.
LESSON 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.

LESSON 4: SIGHT DISTANCE ON
HORIZONTAL CURVES
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.
LESSON 4: SIGHT DISTANCE ON
HORIZONTAL CURVES
Figure 2-4: Sight distance for horizontal
curve (S s L)
Figure 2-4 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.
LESSON 4: SIGHT DISTANCE ON
HORIZONTAL CURVES

Therefore when S < L: ;where R = horizontal curve
S
M
8
2
=
Figure 2-5: Sight
distance on horizontal
curve (S > L)
Figure 2-5 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.
LESSON 4: SIGHT DISTANCE ON
HORIZONTAL CURVES

Therefore when S > L:

Where;
M = desired minimum clearance offset
L = length of curve
R = horizontal curve radius
S = required sight distance
R
L S L
M
8
) 2 (
=
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 s and the coefficient of friction between the tyres and
the road surface is 0.28. Is the suggested offset clearance
adequate to allow for safe stopping sight distance?
LESSON 5: STOPPING SIGHT DISTANCE
ON HORIZONTAL CURVES
The following
shows how
stopping sight
distance (SSD)
on a horizontal
curve can be
calculated given
(R) and middle
ordinate (M).
LESSON 5: STOPPING SIGHT DISTANCE
ON HORIZONTAL CURVES
Based on the diagram:
R L t

A
=
180
length of sight line, l = 2R
v
sin(A
s
/2)
v
s
R SSD t

A
=
180
v
s
R
SSD
t
180
= A
)
2
cos 1 (
A
= R M )
2
cos 1 (
s
v s
R M
A
=
LESSON 5: STOPPING SIGHT DISTANCE
ON HORIZONTAL CURVES
v
s
R
SSD
t
180
= A
)
2
cos 1 (
s
v s
R M
A
=
(
(
(
(

|
|
.
|

\
|
=
2
180
cos 1
v
v s
R
SSD
R M
t
Substitute into
(

|
|
.
|

\
|
=
v
v s
R
SSD
R M
t
90
cos 1
(

|
|
.
|

\
|

=

v
s v v
R
M R R
SSD
1
cos
90
t
LESSON 5: STOPPING SIGHT DISTANCE
ON HORIZONTAL CURVES
Highway Agency Design Speed (km/h) Stopping Sight Distance (m)
Malaysian Highway
Authority (LLM)
140
120
100
80
325
225
150
100
Public Works
Department (JKR)
120
100
80
60
50
40
30
20
285
205
140
85
65
45
20
20
Table 4-3: SSD (JKR & LLM)
LESSON 5: STOPPING SIGHT DISTANCE
ON HORIZONTAL CURVES
Design speed (km/h) Stopping Sight Distance (m)
AASHTO 2001 AASHTO 1994
Design Desirable Minimum
30
40
50
60
70
80
90
100
35
50
65
85
105
130
160
185
29.6
44.4
62.8
84.6
110.8
139.4
168.7
205.0
29.6
44.4
57.4
74.3
94.1
112.8
131.2
157.0
Table 4-4: SSD (AASHTO)
LESSON 5: STOPPING SIGHT DISTANCE
ON HORIZONTAL CURVES
Example 5:

A horizontal curve on a U5 highway is
designed with a 700 m radius, 3.6 m lanes
and a 100 km/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.
LESSON 6: 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.
LESSON 6: SIGHT DISTANCE ON VERTICAL
CURVES
For crest curves;
Figure 2-6: Sight distance on crest vertical curve (S s L)
LESSON 6: SIGHT DISTANCE ON VERTICAL
CURVES
Figure 2-6 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
h
1
= eye height
h
2
= object height
2
2 1
2
min
) 2 2 ( h h
AS
L
+
=
LESSON 6: SIGHT DISTANCE ON VERTICAL
CURVES
Figure 2-7: Sight distance on crest vertical curve (S > L)
Figure 2-7 illustrates the condition where S is greater than
L and overlaps on either sides of the vertical curves.
LESSON 6: SIGHT DISTANCE ON VERTICAL
CURVES

When S > L:

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

(b) When S > L:
A
h h
S L
2
2 1
min
) ( 2
2
+
=
|
.
|

\
|
+

=
2
) ( 8
8
2 1
2
min
h h
D
AS
L
A
h h
D
S L
|
.
|

\
|
+

=
2
) ( 8
8
2
2 1
min
Where;
D = vertical
clearance
(ideally taken
as 5.7 m)
LESSON 6: SIGHT DISTANCE ON
VERTICAL CURVES
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.
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