1. Safety Considerations in Road Geometric Design
Presented at ITS Research Center, Wuhan University of Technology, July 20, 2010
Safety Considerations in Road Geometric Design
Dr. Ming Zhong, P.Eng
UNB Transportation Group
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2. Outline Human factors Sight distance Horizontal alignment design
Minimum radius
Spirals
Superelevation
Lateral clearance
Vertical alignment design
Coordination of horizontal/vertical alignments
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3. (1) Human factors
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4. Goals Understand how to consider human factors in highway design
Expectancy
Empirical value of perception & reaction time (2.5 s)
Understand PIDV process
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5. Major Components of Highway Transportation System
Human (drivers and pedestrian)
Vehicle
Road
Environment
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6. Design Controls Human Factors (TAC 1.2.2) Speed (TAC 1.2.3)
Design Vehicles (TAC 1.2.4)
Sight Distances (TAC 1.2.5)
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7. Roadway design principles
Design a roadway so that it conforms to what drivers expect from such a roadway based on previous experience
Provide drivers with clear clues about what is expected of them on a particular roadway
Limited templates
Philosophy of TAC Manual is to provide a “design domain” rather than a single number
i.e., 150 ≤ k ≤ 200
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8. Expectancy
Expectancy - forward planning for common situations. Utilizes past experiences to reduce reaction time.
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9. Information Presentation to drivers
Primacy
Limited
Priority based
Spread
Expected
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10. Perception plus reaction time
PIDV Process
Perception
Identification
Decision
Volition
Total time: 0.5 to 3.0 seconds
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11. Perception and Reaction Time Design Domain
Perception and Reaction Time (s)
Applicability
0.5 -2
Reaction of alerted drivers to simple stimulus
2.5*
Typically used as being representative of the 90th percentile of drivers and situations
Reaction of unalerted drivers to complex or inconspicuous stimuli
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12. Statistics: Percentile
The pth percentile is a value such that at most (100p)% of the observations are less than this value and that at most 100(1 − p)% are greater. (p is a value between 0 and 1)
90%
2.5 s
T (s)
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13. Design response criteria
Unexpected, unusual or inconsistent design are avoided or minimized
Freeway exit is on the left side
Predictable behavior is encouraged
Limited design template
Consistency of design and driver behavior
no significant change in design speed along a roadway
Concise and uncertainty-reducing information
Adequate sight distance and margins are allowed for error correction and recovery
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14. Geometric Design Guide for Canadian Roads: Section 1.2.5
(2) Sight Distance
Geometric Design Guide for Canadian Roads: Section 1.2.5
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15. Sight Distance There are four types of sight distance
Stopping Sight Distance (SSD)
Passing Sight Distance (PSD)
Decision Sight Distance (DSD)
Intersection Sight Distance (ISD)
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16. Sight Distance Sight distance means how far you can see ahead.
It is affected by a number of criteria(?):
Road geometry
Physical settings (especially for ISD)
Driver Eye Height
Object Height
The actions you need to take (stop & passing)
Coefficient of Pavement Friction
Perception/Reaction Time
Vehicle Operating Speed
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17. Stopping Sight Distances
SSD=Dpr+Ds
Slope=Deceleration rate = fg
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18. Stopping Sight Distance
Distance traveled during perception reaction time
V = velocity in km/hr
v = velocity in m/s
t = reaction time in seconds
f = friction factor
g = acceleration of gravity (9.81m/s2)
Distance traveled during braking time
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19. TAC SSD for Auto and Trucks with ABS (Table 1.2.5.3)
Design speed (km/h)
SSD (meters)
Perception and reaction time (s)
Distance (m)
Friction coefficient
Braking distance
SSD (rounded) 40
2.5
27.8
0.38
16.6 50
0.35
60-65 60
0.33
75-85 ….
…..
130
0.28
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20. TAC SSD for Auto and Trucks with CBS (Table 1.2.5.4)
DS (km/h)
SSD with ABS
SSD with CBS 40 45
60-70 50
60-65
85-110 60
75-85 ….
130
Not Available
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21. Stopping Sight Distance (SSD)
Table gives SSD for various design speed (level grade and wet pavement);
Used for a number of design tasks:
Horizontal/vertical curve design
Intersection geometry
Placement of traffic control devices
Influenced by variation in truck braking systems and pavement conditions
Conventional
Antilock
Dry/wet pavement
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22. Passing Sight Distance
Purpose: to provide passing opportunity over a segment of 2-lane, 2-way highway
Striping
Indicating whether you are allowed to pass
TAC based on AASHTO Studies
1930’s study – very conservative
Advances in vehicle technology etc.
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23. Passing Sight Distance
Is composed of four components
d1: initial maneuver distance - P+R+Acceleration
d2: Distance traveled in left lane
d3: Buffer
d4: Distance traveled by opposing vehicle
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24. Passing Sight Distance
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25. Required PSD in comparison with AASHTO criteria
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26. Decision Sight Distance
SSD is usually inadequate when:
drivers must make complex decisions
Information is hard to find
Information is unusual
Unusual maneuvers are required
Decision sight distance provides addition room (time) to maneuver or come to a stop.
DSD >> SSD
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27. Decision Sight Distance
May be required at:
Complex Intersections / Interchanges
Locations where unusual maneuvers occur
Locations where significant changes to the cross section are made
Complicate areas requiring multiple demands on driver’s decision-making resulting from road elements, traffic control devices etc.
Construction zones
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28. TAC Decision Sight Distances
Design speed (km/h)
TAC DSD (meters) A B C D E 50 75
160
145
200 60 70
125
250
240
275 ….
120+
305
505
375
415
470
Maneuver A: Stop on rural road
Maneuver B: Stop on urban road
Maneuver C: Speed/path/direction change on rural road
Maneuver D: Speed/path/direction change on suburban road
Maneuver E: Speed/path/direction change on urban road
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29. Differences between TAC and AASHTO DSD ?
Design speed (km/h)
AASHTO DSD (meters) A B C D E 50 70
155
145
170
195 60
115
235
200
275
130
305
525
390
450
510
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30. AASHTO DSD Formulations (FYI)
For A (stop on rural road) and B (stop on urban road) group:
For C, D, and E group (speed/path/direction change on rural, suburban, and urban road):
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31. Intersection Sight Distance
Definition: the sight distance available from a point where vehicles are required to stop on the intersecting road,
while drivers are looking left and right along the major roadway, before entering the intersection.
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32. Sight triangle Approach sight triangle Departure sight triangle
For intersections with no traffic control or yield control
Require to provide stopping sight distance (minimum)
Departure sight triangle
For stop controlled and signalized intersections
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33. Approach sight triangles
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34. Departure sight triangles - crossing
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35. Departure sight triangles – left-turn
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36. Departure sight triangles – right turn
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37. Midway Questions ?
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38. (3) Horizontal Alignment Design
TAC Manual Section 2.1.2
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39. Horizontal Alignment
Horizontal alignment is observed in plan view and consists of:
Tangents;
Curves;
Spirals;
May or may not
Superelevation
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40. Design Elements for Horizontal Alignment
Circular curves
Spiral curves
Compound and reverse curves
Development of superelevation
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41. (3.1) Minimum radius
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42. The minimum radius The centrifugal force is:
Where:
W – the weight of vehicle
ac - acceleration for curvilinear motion
u – operating speed R – curve radius
g – acceleration of gravity
When the vehicle is in equilibrium:
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43. Forces on cornering vehicle
This picture shows you that the forces act on a vehicle running on a superelevated horizontal curve.
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44. Minimum Radius Curve
Typically, a jurisdiction has a maximum allowable superelevation, a maximum fL can be established for given design speed;
kph m
3.6*3.6*9.81
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45. Maximum lateral friction coefficients
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46. minimum radius for rural and high-speed urban roadways1 2 3 4
An object located near the inside edge of the road may interfere with the view of the driver, which results in reduced driver’s sight distance.
In this case, it is necessary to design a horizontal curve such that the available sight distance is at least equal to the safe stopping distance (SSD). That means that a larger radius may have to be used to provide enough SSD. Similar approaches are used to determine the minimum radius as that used for simple curve.
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47. (3.2) Spirals
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48. Spiral Curves Used to connect tangents and simple curves
R changes from infinity to Rcurve to provide a gradual roadway change
Notice the relative scale of simple curve design and spiral design
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49. Why use spirals?
Natural Driving path: reduce off tracking, increase driver comfort;
Centrifugal force increase/decrease gradually
Minimizes encroachment
Promotes speed uniformity (increase safety)
Super-elevation is developed over spiral
and widening if necessary
Enhanced appearance (no breaks in alignment)
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50. Applicability Most Rural roads Urban roads
New facility with superelevations
Can be avoided
Very flat horizontal curve
Simple curves can be applied - low speed facility
Urban roads
Applicable to arterials, expressways, and freeways
With design speed > 70 km/h and superelevation
Interchange ramps
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51. (3.3) Superelevation
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52. Superelevation
If f = 0, all resistance to centripetal forces is provided by “e”
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53. Superelevation Lateral friction counterbalancing centrifugal forces;
fL used in design should be lower than maximum available;
As fLmax is reached, slipping begins;
e also counterbalances some of the centrifugal forces.
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54. How superelevation develops?
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55. Superelevation Rural: Urban:
Jurisdictions in Canada set a maximum on the allowable rate of superelevation (4%, 6%, 8%)
Recommended maximum 6%
Urban:
Local – normal crown (2%)
Collector – 2-4%
Arterial – 6%
Expressways and freeways and ramps – 6-8%
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56. Superelevation
Higher the superelevation rate, the lower the minimum curve radius;
8% not used in rural areas as sharp curve not expected.
Avoid e > 0.04 at existing or future intersections;
High e can cause sideslip at low speeds under icy conditions;
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57. Superelevation development for two scenarios: with or without spirals
Scenario 2 for spiral curve
Scenario 1 for simple curve
Superelevation development for two scenarios: with or without spirals
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58. Superelevation development for simple curves (without spirals)
Super-elevation runoff developed 60% over the tangent, 40% over the curve
Total length equals to required spiral curve
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59. How super-elevation is developed for simple curves?
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60. (3.4) Lateral clearance
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61. Lateral Clearance
Providing sight distance around inside lanes of sharp curves on 2-L, 2-W highways;
Formula assumptions
where SSD or PSD < Curve Length
When vehicle, object, and visual obstacle are all within the curve
Assumes no reduction of lateral friction on curve (will be adjusted later)
Assumes a 0% grade (will be adjusted later).
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62. Typical SSD and PSD values
SSD ranges from 30 m for DS=30 km/h to about 330 m for DS=130 km/h
PSD ranges from 345 m for DS=30 km/h to about 855 m for DS=130 km/h
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63. What clearance we need for a DS and R?
C is the distance from the centerline of inside lane to the obstacle
What clearance we need for a DS and R?
DS determine the stopping or passing sight distance required: S in the equation
C = R – R x cos(∆/2)
S/∆ = 2πR/360
∆=180S/ πR
∆/2=90S/ πR (degree)
= S/(2R) radians
Pay attention to the unit of the angle required in your calculator (in degree) or spreadsheet (in radians).
(Right now ∆ is in degree)
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64. (4) Vertical alignment design
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65. Crest Vertical Curves
Minimum K given for design based on sight distances
K dependant on
drivers eye height (Usually 1.05m)
Height of Object (Depends on type of sight distance required)
SSD - .38m or 0.0m
DSD - .15m
PSD – 1.30m
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66. Minimum length of a crest curve
Where S is either stopping, passing or decision sight distance
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67. minimum SSD and K for crest curves
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68. Minimum PSD and K for crest curves
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69. Sag Vertical Curves
Sag curves do not have sight distance issues by their very nature in the daytime.
Minimum K values based on
comfort (Urban lighted segment)
night time visibility (rural, unlighted segment)
K dependant on
Headlight height (0.6 m)
Headlight orientation (1º upward)
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70. Minimum length of a sag curve
For illuminated roadways
(comfort criterion controls)
For non-illuminated roadways
(lighted sight distance controls)
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71. Minimum SSD and K for sag curves
You can say in general “Headlight Control” governs!
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72. (6) Coordination Rules of Thumb
Curvature, grades and tangents in proper balance/mixture;
Excessive curvature with flat grades
Tangent alignment or flat curve at the expense of steep or long grades
Superimpose vertical and horizontal curvature;
Similar curve lengths used for both > than minimum
Rather than small vertical/horizontal curve overlaid on large horizontal/vertical curve
Avoid sharp horizontal curvature at the high or low point of pronounced vertical curvature;
Don’t forget passing opportunities (on TLTW);
It may supersede the desirability of “coordination and aesthetics”
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73. Questions ?
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