Sight Distance AMRC 2012 MODULE 7 CONTENTS

Transcription

1 AMRC 2012 MODULE 7 Sight Distance CONTENTS Overview Objectives Procedures Introduction Stopping Sight Distance Passing Sight Distance Decision Sight Distance Self-Test WPC # /09

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3 Module 7 Sight Distance Overview This module explains one of the most critical design elements, sight distance. Sight distance is the criterion that determines vertical and horizontal alignment. It deals with driver s perception and reaction while driving, as well as the driving environment. The sight distance concept is well documented in the Geometric Design Guide for Canadian Roads by the Transportation Association of Canada (TAC Manual) and the AASHTO s policy on Geometric Design of Highways and Streets (AASHTO Policy). You are encouraged to read the relevant sections of these references. Objectives Upon completion of this module, the student will be able to: explain the concept of sight distances describe stopping sight distance describe passing sight distance describe decision sight distance. Procedures Study the module materials and make notes as required. Perform the self-test on these principles and review the course materials in such a manner as to be able to successfully complete similar questions upon examination. WPC # /09 AMRC

4 MODULE 7 Sight Distance 7-2 AMRC 2012 WPC # /09

5 SECTION 7.1 Introduction Drivers receive about 90 percent of their highway information visually. The sight distance is, therefore, a fundamental element in highway design. Three sight distances are currently used in design: stopping, passing, and decision. The TAC design manual (TAC Manual) identifies the criteria used in calculating sight distance: perception-reaction time driver eye height vehicle operating speed object height coefficient of pavement friction. Perception time is a time during which the brain interprets sensory information. It includes the intellectual time for reasoning, problem solving, and decision making. Reaction time is the time taken for the brain to send instructions to the body and for the body to execute the instruction. The driver perception-reaction time normally ranges between 0.5 seconds to 4.5 seconds. Perception-reaction time less than 2 seconds would apply to alert drivers in simple conditions; 2.5 seconds to average drivers under average conditions; 4.5 seconds to unalert drivers in complex conditions (TAC Manual). The average vehicle height of passenger vehicles has decreased since the 1960s. The average eye height had also decreased correspondingly. The change in eye height has the effect of lengthening minimum crest curves by approximately 5 percent, thereby providing about 2.5 percent more sight distance (AASHTO Policy). The TAC design manual recommends a design eye height of 1.05 m for passenger vehicles, 1.8 m for buses, and 2.1 m for large trucks. Usually passenger vehicle eye height is used in design since a larger vehicle would allow the driver to see further away, thereby reducing the sight distance. The size of an object on the road to be discernable to drivers is the next consideration. The TAC design manual recommends an object height of 0.15 m. This height represents the lowest object that can create a hazardous condition to be seen by drivers (AASHTO Policy). WPC # /09 AMRC

6 MODULE 7 Sight Distance The pavement friction coefficient changes with weather conditions. The braking distance, therefore, varies depending on pavement conditions, conditions of the tires, vehicle speed, and the gradient of the road. A typical friction coefficient for wet pavement for a vehicle speed of 80 km/h is You may refer to publications (Chapter 3, AASHTO Policy) for other values. 7-4 AMRC 2012 WPC # /09

7 SECTION 7.2 Stopping Sight Distance Stopping sight distance is the sum of the distance travelled during the perception and reaction time plus the braking distance. The TAC manual provides an equation for the calculating of the Stopping Sight Distance (SSD). The general equation given in the TAC Manual is: SSD = (perception and reaction time) + braking distance 2 V SSD 0.278tV 254(f G ) Where: SSD = Stopping sight distance, m V = Initial speed, km/h t = Perception/reaction time, 2.5 seconds f = Coefficient of friction G = Percent of grade divided by 100, + for phill and for downhill See equations and in the TAC manual. The first term 0.278t is the distance travelled during the perception/reaction time. The second term is the distance required for braking. The term G accounts for the gradient, since uphill braking distance will be less than that of a downhill gradient. Section of the TAC design manual and Chapter 3 of AASHTO Policy provide an insight of the stopping sight distance. WPC # /09 AMRC

8 MODULE 7 Sight Distance 7-6 AMRC 2012 WPC # /09

9 SECTION 7.3 Passing Sight Distance On two-lane rural roads, when there is sufficient passing sight distance, traffic is allowed to pass slow moving vehicles by using the lane for opposing traffic. The passing zone is generally indicated by centreline marking (broken line) as shown in Figure 7.1. The passing sight distance consists of four components: D1 Initial perception/reaction and manoeuvre distance to get into position to enter the opposing lane. D2 Distance travelled on the opposing lane during passing manoeuvre. D3 Distance between the opposing vehicle and the passing vehicle at the end of the passing manoeuvre. D4 Distance travelled by the opposing vehicle during the passing manoeuvre, or two thirds of D2. Source: Transportation Association of Canada. (1999). Geometric Design Guide for Canadian Roads, Part 1. Ottawa: Transportation Association of Canada. p Figure 7.1 Passing Sight Distance WPC # /09 AMRC

10 MODULE 7 Sight Distance The distances D1, D2, D3, and D4 are usually given in table form. Section of the TAC design manual shows the minimum passing sight distance for various speeds. Chapter 3 of the AASHTO Policy shows the detailed calculation requirements and description as well. 7-8 AMRC 2012 WPC # /09

11 SECTION 7.4 Decision Sight Distance In a complex driving environment, decision time is longer than the time required by the stopping sight distance. If the design is to allow for less alert and competent drivers, decision sight distance may be used. Decision sight distance is the distance required for a driver to detect an unexpected or otherwise difficult-to-perceive information source or hazard in a roadway environment that may be visually cluttered, recognize the hazard or its potential threat, select an appropriate speed or path, and initiate and complete the required safety manoeuvre safely and efficiently (AASHTO Policy). Drivers need decision sight distance whenever there is likelihood for error in either information reception, decision making, or control actions (AASHTO Policy). The follow equation can be used for calculating decision sight distance: DSD = VD (T1 + T2 + T3) Where: DSD = Decision sight distance, m VD = Design speed, m/sec T1 = Detection and recognition time, seconds T2 = Decision and response time, seconds T3 = Manoeuvre time, seconds Section of the TAC design manual shows the minimum decision sight distance for various speeds. Chapter 3 of the AASHTO Policy shows the detailed calculation requirements and description as well. WPC # /09 AMRC

12 MODULE 7 Sight Distance 7-10 AMRC 2012 WPC # /09

13 SECTION 7.5 Self-Test Provided below is a self-test for this module. Perform the test and compare your answers with those given in your student guide. If you have difficulty with the self-test, review the course materials and make notes in order to be able to successfully complete similar questions upon examination. 1. What are the five criteria used in calculating sight distances? 2. What is SSD? Describe the two components in the SSD equation. 3. What is PSD? Describe the four components in the PSD equation. 4. What is DSD? Describe the components in the DSD equation. WPC # /09 AMRC

14 MODULE 7 Sight Distance 7-12 AMRC 2012 WPC # /09