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 道路设计中的安全问题（1） 2018/12/2

Outline Human factors Sight distance Horizontal alignment design Minimum radius Spirals Superelevation Lateral clearance Vertical alignment design Coordination of horizontal/vertical alignments 道路设计中的安全问题（1） 2018/12/2

(1) Human factors 道路设计中的安全问题（1） 2018/12/2

Goals Understand how to consider human factors in highway design Expectancy Empirical value of perception & reaction time (2.5 s) Understand PIDV process 道路设计中的安全问题（1） 2018/12/2

Major Components of Highway Transportation System Human (drivers and pedestrian) Vehicle Road Environment 道路设计中的安全问题（1） 2018/12/2

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) 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

Expectancy Expectancy - forward planning for common situations. Utilizes past experiences to reduce reaction time. 道路设计中的安全问题（1） 2018/12/2

Information Presentation to drivers Primacy Limited Priority based Spread Expected 道路设计中的安全问题（1） 2018/12/2

Perception plus reaction time PIDV Process Perception Identification Decision Volition Total time: 0.5 to 3.0 seconds 道路设计中的安全问题（1） 2018/12/2

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 3.0-4.5 Reaction of unalerted drivers to complex or inconspicuous stimuli 道路设计中的安全问题（1） 2018/12/2

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) 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

Geometric Design Guide for Canadian Roads: Section 1.2.5 (2) Sight Distance Geometric Design Guide for Canadian Roads: Section 1.2.5 道路设计中的安全问题（1） 2018/12/2

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) 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

Stopping Sight Distances SSD=Dpr+Ds Slope=Deceleration rate = fg 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

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 32.7-34.7 0.35 24.8-28.1 60-65 60 38.2-41.7 0.33 36.1-42.9 75-85 …. ….. 130 72.9-90.3 0.28 155-237.6 230-330 道路设计中的安全问题（1） 2018/12/2

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 75-85 105-130 …. 130 230-330 Not Available 道路设计中的安全问题（1） 2018/12/2

Stopping Sight Distance (SSD) Table 1.2.5.3 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 道路设计中的安全问题（1） 2018/12/2

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. 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

Passing Sight Distance 道路设计中的安全问题（1） 2018/12/2

Required PSD in comparison with AASHTO criteria 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

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): 道路设计中的安全问题（1） 2018/12/2

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. 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

Approach sight triangles 道路设计中的安全问题（1） 2018/12/2

Departure sight triangles - crossing 道路设计中的安全问题（1） 2018/12/2

Departure sight triangles – left-turn 道路设计中的安全问题（1） 2018/12/2

Departure sight triangles – right turn 道路设计中的安全问题（1） 2018/12/2

Midway Questions ? 道路设计中的安全问题（1） 2018/12/2

(3) Horizontal Alignment Design TAC Manual Section 2.1.2 道路设计中的安全问题（1） 2018/12/2

Horizontal Alignment Horizontal alignment is observed in plan view and consists of: Tangents; Curves; Spirals; May or may not Superelevation 道路设计中的安全问题（1） 2018/12/2

Design Elements for Horizontal Alignment Circular curves Spiral curves Compound and reverse curves Development of superelevation 道路设计中的安全问题（1） 2018/12/2

(3.1) Minimum radius 道路设计中的安全问题（1） 2018/12/2

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: 道路设计中的安全问题（1） 2018/12/2

Forces on cornering vehicle This picture shows you that the forces act on a vehicle running on a superelevated horizontal curve. 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

Maximum lateral friction coefficients 道路设计中的安全问题（1） 2018/12/2

minimum radius for rural and high-speed urban roadways 1 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. 道路设计中的安全问题（1） 2018/12/2

(3.2) Spirals 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

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) 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

(3.3) Superelevation 道路设计中的安全问题（1） 2018/12/2

Superelevation If f = 0, all resistance to centripetal forces is provided by “e” 道路设计中的安全问题（1） 2018/12/2

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. 道路设计中的安全问题（1） 2018/12/2

How superelevation develops? 道路设计中的安全问题（1） 2018/12/2

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% 道路设计中的安全问题（1） 2018/12/2

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; 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

How super-elevation is developed for simple curves? 道路设计中的安全问题（1） 2018/12/2

(3.4) Lateral clearance 道路设计中的安全问题（1） 2018/12/2

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). 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

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) 道路设计中的安全问题（1） 2018/12/2

(4) Vertical alignment design 道路设计中的安全问题（1） 2018/12/2

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 道路设计中的安全问题（1） 2018/12/2

Minimum length of a crest curve Where S is either stopping, passing or decision sight distance 道路设计中的安全问题（1） 2018/12/2

minimum SSD and K for crest curves 道路设计中的安全问题（1） 2018/12/2

Minimum PSD and K for crest curves 道路设计中的安全问题（1） 2018/12/2

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) 道路设计中的安全问题（1） 2018/12/2

Minimum length of a sag curve For illuminated roadways (comfort criterion controls) For non-illuminated roadways (lighted sight distance controls) 道路设计中的安全问题（1） 2018/12/2

Minimum SSD and K for sag curves You can say in general “Headlight Control” governs! 道路设计中的安全问题（1） 2018/12/2

(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” 道路设计中的安全问题（1） 2018/12/2

Questions ? 道路设计中的安全问题（1） 2018/12/2