SIGHT DISTANCE
Presented by
Nazir Lalani P.E.
Traffex Engineers Inc.
[email protected]
WHY IS SIGHT DISTANCE SO
IMPORTANT?
Drivers must be able to:
„
Stop for objects in the roadway
„
Stop for stationary vehicles ahead
„
See cross traffic at intersections before entering
„
See on coming vehicles when passing
„
See traffic control devices and react to them
„
See trains at Railroad Xings not controlled by gates
„
See pedestrians waiting to cross the street
1
AASHTO GEOMETRIC DESIGN BOOK
What is Stopping Sight
Distance?
2
¾Stopping Sight Distance: The available sight
distance on a roadway should be sufficiently
long to enable a vehicle traveling at or near the
design speed to stop before reaching a
stationary object in its path
Stopping sight distance is the sum of two distances:
¾ The distance traversed by the vehicle from the instant
the driver sights an object necessitating a stop to the
instant the brakes are applied (brake reaction
distance).
distance).
Brake Reaction Time
¾ The distance needed to stop the vehicle from the
instant brake application begins (braking distance).
distance).
3
Brake
Reaction
Time
Braking
Distance
Braking Distance
¾ The approximate braking distance of a vehicle on a level
roadway traveling :
US Customary
V²
d = 1.075 a
Where:
d = braking distance;
V = design speed, mph;
a = deceleration rate, ft/s²
ft/s²
4
¾Studies documented in the literature show that most drivers decelerate at
a rate greater than 14.8 ft/s² when confronted with the need to stop for an
unexpected object in the roadway
US Customary
Approximately
90 percent of
all drivers
decelerate at
rates greater
than 11.2 ft/s²
V²
d = 1.075 a
Where:
d = braking distance;
V = design speed, mph;
a = deceleration rate, ft/s²
ft/s²
Therefore,
11.2 ft/s² (a
comfortable
deceleration
for most
drivers) is
recommended
as the
deceleration
threshold for
determining
stopping sight
distance
Brake
Reaction
Time
Brake
Reaction
Distance
5
Brake Reaction Time
¾ Brake reaction time is the interval from the instant that the
driver recognizes the existence of an obstacle on the roadway
ahead that necessitates braking to the instant that the driver
actually applies the brakes
¾ In addition, the driver must not only see the object but must
also recognize it as a stationary or slowly moving object
Brake Reaction Time
¾ Comprises of PIEV which includes speed and conditions.
Perception
Identification (understanding)
Emotion (decision making)
Volition (execution of decision)
6
¾ Both recent research and the studies documented in the
literature show that a 2.52.5-s brake reaction time for stopping sight
situations encompasses the capabilities of most drivers,
including those of older drivers
¾ The recommended design criterion of 2.52.5-s for brake reaction
time exceeds the 90th percentile of reaction time for all drivers
Stopping Sight Distance
¾ The sum of the distance traversed during the brake reaction time and
the braking distance is the stopping sight distance
US Customary
V²
d = 1.47Vt + 1.075 a
¾Driver’s eye is estimated to be 3.5 ft
and the height of the object to be seen
by the driver is 2 ft, equivalent to the
tail light height of a passenger car.
Where:
V = design speed, mph;
a = deceleration rate, ft/s²
ft/s²
t = Brake reaction time in
seconds
7
US Customary
Stopping sight distance
Design
Brake reaction
Braking distance
speed
distance
on level
Calculated
Design
(mph)
(ft)
(ft)
(ft)
(ft)
15
55.1
21.6
76.7
80
20
73.5
38.4
111.9
115
25
91.9
60
151.9
155
30
110.3
86.4
196.7
200
35
128.6
117.6
246.2
250
40
147
153.6
300.6
305
360
45
165.4
194.4
359.8
50
183.8
240
423.8
425
55
202.1
290.3
492.4
495
60
220.5
345.5
566
570
65
238.9
405.5
644.4
645
70
257.3
470.3
727.6
730
75
275.6
539.9
815.5
820
80
294
614.3
908.3
910
Source: Geometric Design of Highways and Streets 2004
Why is it important on
horizontal and vertical
curves?
8
Vertical Curve Crest
Condition
9
Algebraic Difference in Grades (%)
Design Speed (mph)
Exhibit 3-71: Length of Crest Vertical
Curve (feet)
Source: Geometric Design of Highways and Streets 2004
Exhibit 3-72 Design Controls for Crest Vertical
Curves Based on Stopping Sight Distance
Source: Geometric Design of Highways and Streets 2004
10
Exhibit 3-73 Design Controls for Crest Vertical
Curves Based on Passing Sight Distance
Source: Geometric Design of Highways and Streets 2004
Not designed for either stopping or passing sight
distance!
11
Vertical Curve Sag
Condition
Stopping Sight Distance on Sag Vertical
Curves
The minimum length of vertical curve which provides headlight sight
sight
distance in grade sags for a given design speed can be obtained.
Source: Caltrans Highway Design Manual
12
Algebraic Difference in Grades (%)
Design Speed (mph)
Exhibit 3-74 Length of Sag Vertical Curve (feet)
Source: Geometric Design of Highways and Streets 2004
Exhibit 3-75 Design Controls for Sag Vertical Curves
Based on Stopping Sight Distance
Source: Geometric Design of Highways and Streets 2004
13
Formula for length of sag vertical curve using
comfort factor ( length is 50% less than based on
headlight distance)
Exhibit 3-76 Sight Distance at Under
Crossings
Note: AASHTO provide different
formulas for calculating curves for under Crossings
Source: Geometric Design of Highways and Streets 2004
14
Bridge structure limits
sight distance
General Controls for Vertical Alignments
„
Provide smooth grade line with gradual changes
„
Avoid “roller coaster” or sudden dip type profiles
„
Avoid “broken back” curves ( two vertical curves in
the same direction separated by short tangent
„
On long grades, the steepest at the bottom with
flattening of the grades near the top
„
Sag vertical curves must have adequate drainage
15
Sight Distance on
Horizontal Curves
Stopping Sight Distance on Horizontal Curves
¾Where an object off the pavement such as a bridge pier, building,
building,
cut slope, or natural growth restricts sight distance, the minimum
minimum
radius of curvature is determined by the stopping sight distance.
distance.
¾HSO: Horizontal
Sightline Offset
Insert Exhibit 3-54
¾Available stopping
sight distance on
horizontal curves is
obtained from Exhibit
3-53
¾It is assumed
that the driver’
driver’s
eye is 3.5 feet
above the center of
the inside lane
(inside
with
respect to curve)
and the object is 2
feet high
Source: Geometric Design of Highways and Streets 2004
16
Source: Geometric Design of Highways and Streets 2004
General Controls for Horizontal Alignments
„
Provide passing distance on 2-lane roads
„
Provide greater radius of curvature than the
minimum where possible
„
Avoid sharp horizontal curves at the ends of long
tangent sections and back to back reverse curves
„
Curves should be at least 500 feet for a central
angle of 5 degrees – 100 feet per degree
„
Minimum length of horizontal curves should be 15
times the design speed in mph
17
Stopping Sight Distance for
Bicyclists
Source: Caltrans Highway Design Manual Chapter 1000
18
Source: Caltrans Highway Design Manual Chapter 1000
Minimum Length of Crest Vertical Curve
Source: Caltrans Highway Design Manual Chapter 1000
19
Horizontal Lateral Clearance Formula
Source: Caltrans Highway Design Manual Chapter 1000
Stopping Sight Distance
Source: Caltrans Highway Design Manual Chapter 1000
20
Intersection Sight Distance
Uncontrolled Intersections
21
Sight Triangles
¾ Specified areas along intersection approach legs and across
their included corners should be clear of obstructions that might
block a driver’s view of potentially conflicting vehicles
¾ These specified areas are known as clear sight triangles
¾ The dimensions of the legs of the sight triangles depend on the
design speeds of the intersecting roadways and the type of traffic
traffic
control used at the intersection.
¾ Two types of clear sight triangles are considered in intersection
design:
Approach Sight
Triangles
A
N
D
Departure Sight
Triangles
22
Approach Sight
Triangles for
Uncontrolled
Locations
¾ Each quadrant of an intersection should contain a triangular area
area
free of obstructions that might block an approaching driver’
driver’s view
of potentially conflicting vehicles - drivers eye height and object
height are 3.5 feet (AASTO)
¾ The length of the legs of this triangular area, along both
intersecting roadways, should be such that the drivers can see any
any
potentially conflicting vehicles in sufficient time to slow or stop
stop
before colliding within the intersection
Exhibit 9-50: Intersection Sight Triangles
Minor Road
¾ This decision point is the location at
which the minorminor-road driver should begin
to brake to a stop if another vehicle is
present on an intersecting approach.
b
Major Road
a
Clear Sight Triangle
Decision Point
¾ The distance from the major road,
along the minor road, is illustrated by the
dimension “a” in Exhibit 99-50 A.
¾ Dimension “b” illustrates the length of
this leg of the sight triangle along the
major road A.
A- Approach Sight Triangles
(uncontrolled)
Source: AASHTO A Policy on Design of Highways and Streets
23
Corner sight triangle for uncontrolled locations
Source: AASHTO A Policy on Design of Highways and Streets
Stop Sign Controlled
Intersections
24
Exhibit 9-50: Intersection Sight Triangles
Minor Road
¾ Although desirable at higher volume
intersections, approach sight triangles
like those shown in exhibit 9-50A are
not needed for intersection approaches
controlled by stop signs or traffic
signals.
b
Major Road
a
Clear Sight Triangle
¾ In that case, the need for
approaching vehicles to stop at the
intersection is determined by the traffic
control devices and not by the presence
or absence of vehicles on the
intersecting approaches.
Decision Point
A- Approach Sight Triangles
(uncontrolled or yieldcontrolled)
Source: AASHTO A Policy on Design of Highways and Streets
Case B-Intersections with Stop Control on
the Minor Road
¾ Departure sight triangles for intersections with stop control on
the minor road should be considered for three situations:
• Case B1---Left turns from the minor road;
• Case B2---Right turn from the minor road; and
• Case B3---Crossing the major road from a minor-road approach
¾ Intersection sight distance criteria for stop-controlled
intersections are longer than stopping sight distance to ensure that
the intersection operates smoothly
¾ Minor-road vehicle operators have to wait for a gap in cross
traffic until they can proceed safely without forcing a major-road
vehicle to stop
25
Exhibit 99-50. Intersection
Sight Triangles
Minor Road
¾ A second type of clear sight triangle
provides sight distance sufficient for a
stopped driver on a minor-road approach to
depart from the intersection and enter or
cross the major road.
b
Major Road
a
Decision Point
Clear Sight Triangle
¾ Departure sight triangles should be
provided in each quadrant of each
intersection approach controlled by stop or
yield signs.
B- Departure Sight Triangles
(Stop control)
Source: AASHTO A Policy on Design of Highways and Streets
26
Exhibit 99-50. Intersection Sight
Triangles
Minor Road
¾ Departure sight triangles should also be
provided for some signalized intersection
approaches where right turns on red are
permitted.
b
Major Road
a
Decision Point
Clear Sight Triangle
¾ The recommended dimensions of the clear
sight triangle for desirable traffic operations
where stopped vehicles enter or cross a major
road are based on assumptions derived from
field observations of driver gap acceptance
behavior.
B- Departure Sight Triangles
(Stop control)
Source: AASHTO A Policy on Design of Highways and Streets
Case B1---Left Turn from the Minor Road
¾ The vertex (decision point) of the departure sight triangle on
the minor road should be 4.4 m [14.5 ft] from the edge of the
major-road traveled way
¾ This represents the typical position of the minor-road driver’s
eye when a vehicle is stopped relatively close to the major road
Minor Road
b
Major Road
a
Exhibit 9-50B:
Intersection Sight
Triangles
Decision Point
Clear Sight Triangle
Departure Sight Triangles
Source: AASHTO A Policy on
Design of Highways and Streets
(Stop control)
27
„
The intersection sight distance along
the major road (dimension “b” in Exhibit
9-50B) is determined by:
US Customary
ISD = 1.47 V major t
g
(9-1)
where:
ISD
= intersection sight distance (length of the leg of sight
triangle along the major road) (ft)
V major
= design speed of major
tg
= time gap for minor road
road (mph)
vehicle to enter the major
road (s)
Source: AASHTO A Policy on Design of Highways and Streets
Exhibit 9-54. Time Gap for Case B1---Left Turn from Stop
Time gap (tg) (seconds) at design speed
Design vehicle
Passenger car Single‐
Single‐unit truck Combination truck of major road
7.5
9.5
11.5
Note: Time gaps are for a stopped vehicle to turn left onto a two
two-lane highway with no median and
grades 3 percent or less. The table values require adjustment as follows:
For multilane highways:
For left turns onto twotwo-way highways with more than two lanes, add 0.5 seconds for
passenger cars or 0.7 seconds for trucks for each additional lane,
lane, from the left, in excess of
one, to be crossed by the turning vehicle
For minor road approach grades:
If the approach grade is an upgrade that exceeds 3 percent; add 0.2 seconds for each
percent grade for left turns
Source: AASHTO A Policy on Design of Highways and Streets
28
NOTE:
„
Where substantial volumes of heavy vehicles enter the major
road, such as from a ramp terminal, the use of tabulated values
for singlesingle-unit or combination trucks should be considered.
„
Adjustment for the grade of the minorminor-road approach is needed
only if the rear wheels of the design vehicle would be on an
upgrade that exceeds 3 percent when the vehicle is at the stop
line of the minorminor-road approach.
„
Use the tabulated values shown in Exhibit 99-55 from AASTHO if
no adjustments are needed.
Exhibit 9‐
Exhibit 9‐55. Design Intersection Sight Distance—
55. Design Intersection Sight Distance—Case B1—
Case B1—Left Turn Stop
Design
Speed
(mph)
15
20
25
30
35
40
45
50
55
60
65
70
75
80
US Customary
Intersection sight
distance for
Stopping
passenger cars
sight
distance
Calculated
Design
(ft)
(ft)
(ft)
80
165.4
170
115
220.5
225
155
275.6
280
200
330.8
335
250
385.9
390
305
441.0
445
360
496.1
500
425
551.3
555
495
606.4
610
570
661.5
665
645
716.6
720
730
771.8
775
820
826.9
830
910
882.0
885
Note: Intersection sight distance shown is for a stopped passenger Note: Intersection sight distance shown is for a stopped passenger car to turn left onto a two‐
car to turn left onto a two‐lane highway with no median and grades 3 percent or less..
Source: Geometric Design of Highways and Streets 2004
29
Motorcyclist
Motorcyclist- Left-turning Vehicle crash
30
Case B2—Right turn from the Minor Road
¾ The intersection sight distance for right turns is determined in
the same manner as for Case B1, except that the time gaps (tg) in
Exhibit 9-54 should be adjusted
¾ Field observations indicate that, in making right turns, drivers
generally accept gaps that are slightly shorter than those accepted
in making left turns
¾ The time gaps in Exhibit 9-54 can be decreased by 1.0 s for rightturn maneuvers without undue interference with major-road traffic
„
The intersection sight distance along the major road (dimension “b” in Exhibit 9‐50B) is determined by:
US Customary
ISD = 1.47 V major t
g
(9-1)
where:
ISD
= intersection sight distance (length of the leg of sight
triangle along the major road) (ft)
V major
= design speed of major
tg
= time gap for minor road
road (mph)
vehicle to enter the major
road (s)
Source: AASHTO A Policy on Design of Highways and Streets
31
Exhibit 9-57. Time Gap for Case B2---Right Turn from Stop
and Case B3---Crossing Maneuver
Time gap (tg) (seconds) at design speed
Design vehicle
of major road
Passenger car Single‐
Single‐unit truck Combination truck 6.5
8.5
10.5
Note: Time gaps are for a stopped vehicle to turn right onto or cross a twotwo-lane highway with no
median and grades 3 percent or less. The table values require adjustment
adjustment as follows:
For multilane highways:
For crossing a major road with more than two lanes, add 0.5 seconds
seconds for passenger cars and
0.7 seconds for trucks for each additional lane to be crossed and
and for narrow medians that
cannot store the design vehicle
For minor road approach grades:
If the approach grade is an upgrade that exceeds 3 percent; add 0.1 seconds for each
percent grade
Source: AASHTO A Policy on Design of Highways and Streets
Exhibit 99-58. Design Intersection Sight Distance—
Distance—Case B2—
B2—Right Turn from Stop and
Case B3—
B3—Crossing Maneuver
Design
Speed
(mph)
15
20
25
30
35
40
45
50
55
60
65
70
75
80
US Customary
Intersection sight
distance for
Stopping
passenger cars
sight
distance
Calculated
Design
(ft)
(ft)
(ft)
80
143.3
145
115
191.1
195
155
238.9
240
200
286.7
290
250
334.4
335
305
382.2
385
360
430.0
430
425
477.8
480
495
525.5
530
570
573.3
575
645
621.1
625
730
668.9
670
820
716.6
720
910
764.4
765
Note: Intersection sight distance shown is for a stopped passenger
car to turn right onto or cross a two-lane highway with no median and
grades 3 percent or less. For other conditions, the time gap must be
adjusted and required sight distance recalculated.
Source: Geometric Design of Highways and Streets 2004
32
Case B3—Crossing the Major Road
from a Minor-road approach
¾ In most cases, the departure sight triangles for left and right turns onto
the major road, as described for cases B1 and B2, will also provide more
than adequate sight distance for minor-road vehicles to cross the major
road
¾ However, in the following situations, it is advisable to check the
availability of sight distance for crossing maneuvers:
¾ Where left and/or right turns are not permitted from a particular approach and the
crossing maneuver is the only legal maneuver;
¾ Where the crossing vehicle would cross the equivalent width of more than six lanes;
or
¾ Where substantial volumes of heavy vehicles cross the highway and steep grades
that might slow the vehicle while its back portion is still in the intersection are present
on the departure roadway on the far side of the intersection
Yield Controlled Intersections
33
Yield Controlled Intersections
„
Exhibits 9-60 to 9-64 in AASHTO address Yield
controlled intersections
„
Assumes vehicle will slow but not stop
„
Experience shows that drivers tend to treat them as
uncontrolled intersections and do not slow for
through movements
„
AASHTO assumes drivers turning left or right will
slow to 10 mph
Exhibit 9-61: Case C1 – Length of Sight Triangle for
Crossing Maneuvers at Yield Controlled
Intersections
34
Exhibit 9-64: Case C2 – Design Intersection Sight
Distance for Left/Right Turns Yield Controlled
Intersections
Decision Sight Distance
35
Exhibit 3-3: Decision Sight Distance
Design Values
¾ The sum of the distance traversed during the brake reaction time and
the distance to brake the vehicle to a stop is the stopping sight
sight distance
US Customary
V²
d = 1.47Vt + 1.075 a
¾Driver’
Driver’s eye is estimated to be 3.5 ft
and the height of the object to be seen
by the driver is 2 ft, equivalent to the
tail light height of a passenger car.
Where:
V = design speed, mph;
a = deceleration rate, ft/s²
ft/s²
t = Brake reaction time in
seconds
36
What Causes Corner Sight
Distance Obstructions?
Corner Sight Distance
Obstructions
„
Parked Vehicles
„
Vegetation
„
Horizontal and vertical curves
„
Signs
„
Offset curbs
37
Vegetation Obstruction Before
Vegetation Obstruction After Trimming
38
Vegetation Obstruction Before
Vegetation Obstruction After
39
Vertical Curve Restriction
Corner sight distance blocked by sign
40
Corner sight distance blocked by parking
41
Sight distance obstruction caused
by newspaper racks
Now trees have been added to the mix
42
Source: MUTCD
Sign and limit line
placement at intersections
Manual on Uniform Traffic
Control Devices (MUTCD)
National MUTCD
National standard for
all traffic control
devices installed on
any street, highway
or
bicycle trail open to
public travel.
Web site:mutcd.fhwa.dot.gov
Compliance dates:http://mutcd.fhwa.dot.gov/kno-compliance.htm
43
Source: Public Works Magazine, July 1981
Intersection looking left from a well
position stop limit line
44
Left
-turn Sight Distance for
Left-turn
Traffic Turning from Major
Street
Sight distance triangles at driveways
Reproduced with permission of the Transportation Research Board (TRB)
from the Access Management Manual, TRB, Washington DC, 2003
45
Transportation Research Board (TRB)
Access Management Manual, TRB,
Washington DC, 2003
Left-turn sight distance at
driveway blocked by trees
46
Exhibit 9-67 - Intersection Sight Distance Left-turn from the Major Road
Source: AASHTO Policy on Design of Highways and Streets
Exhibit 99-54.
-54. Time Gap for Case F--F---Left
Left Turns from Major Rd
9
F---Left
Time gap (tg) (seconds) at design speed
Design vehicle
Passenger car Single‐
Single‐unit truck Combination truck of major road
5.5
6.5
7.5
For multilane highways:
For left turning vehicles that cross more than one opposing lane,
lane, add 0.5 seconds for
passenger cars or 0.7 seconds for trucks for each additional lane
lane to be crossed by the turning
vehicle
Source: AASHTO A Policy on Design of Highways and Streets
47
Left-turn sight distance blocked by trees
Talk about protected permissive left turns
Passing Sight Distance
48
No passing Zone on
Horizontal Curve
Passing Sight
Distance
¾ Passing sight distance is considered only on 22-lane
roads
¾ At critical locations, a stretch of 33- or 44-lane passing
section with stopping sight distance is sometimes
more economical than two lanes with passing sight
distance
49
Dashed yellow center line on downhill
side of this up hill passing lane!
Passing Sight Distance - AASHTO
¾ The sight distance available for passing at any place is the
longest distance at which a driver whose eyes are 3.5 feet above
the pavement surface can see the top of an object 3.5 feet high
on the road
¾ In general, 22-lane highways should be designed to provide for
passing where possible, especially those routes with high
volumes of trucks or recreational vehicles
¾ Passing should be done on tangent horizontal alignments with
constant grades or a slight sag vertical curve
¾ Minimum passing sight distance is about four times the
minimum stopping sight distance at the same design speed
50
Passing Sight Distance - AASHTO
¾ Passing sight distance is
the minimum sight distance
required for the driver of one
vehicle to pass another vehicle
safely and comfortably at a 10
mph speed differential
¾ Passing must be
accomplished assuming an
oncoming vehicle comes into
view and maintains the design
speed, without reduction after
the overtaking maneuver is
started
Sight Distance Standards
Design Speed
(mph)
20
25
30
35
40
45
50
55
60
65
70
75
80
Stopping
(ft)
125
150
200
250
300
360
430
500
580
660
750
840
930
Passing
(ft)
800
950
1100
1300
1500
1650
1800
1950
2100
2300
2500
2600
2700
Minimum Passing Sight Distances - MUTCD
¾ MUTCD Distances are shorter because the assumed
difference in speed is greater
51
Pedestrians
Clear sight distance should be provided on
approaches to a crosswalk
52
Clear sight distance based on the gap time needed for a
pedestrian to cross the street should be provided on the
approaches to a crosswalk
Traffic Signals
53
Cone of Vision - MUTCD
Can you spot the signal?
54
Railroad Xings
55
http://www.ite.org/bookstore/gradecrossing/lo_res_RR_BOOK.pdf
56
Sight Distance for Roundabouts
Stopping Sight Distance
Source: FHWA Roundabout Guide
57
Source: Modern Roundabouts for Oregon (ODOT)
Source: WA DOT Design Manual – Chapter 915
58
Source: Caltrans Design Information Bulletin 8080-01:
Source: Caltrans Design Information Bulletin 8080-01
59
(6.5 seconds)
Roundabout stopping sight distance
Source: Caltrans Design Information Bulletin 8080-01: Roundabouts
Source: Modern Roundabouts for Oregon (ODOT)
60
QUESTIONS ?
61