chapter 2 basic components in traffic engineering

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					Chapter 2

Basic Components in Traffic Engineering
Road users including drivers, cyclists and pedestrians Vehicles Roads and highways Control devices

2.1 Road User Characteristics: Perception-reaction time (PRT, 反应时间) --- The perception time includes the detection, identification, and decision elements involved in responding to a stimulus, whereas the reaction time is the time it takes to initiate the physical response. Some references refer it to the perception identification emotion volition time or PIEV, which is comparable in concept to the PRT. It varies with the complexity of the task, whether the event is expected or unexpected, and factors affecting human characteristics. MUTCD recommend PIEV = 3 to 10 seconds; AASHTO suggests a PRT of 2.5 seconds. (see Figure 2.1)
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Information Content (Bits) (a) Median driver reaction time to expected and unexpected information

Information Content (Bits) (b) 85th percentile driver reaction time to expected and unexpected information

Figure 2.1 Perception-reaction times from the AASHTO Green Book Visual Acuity --- including static visual acuity (静视力), the

ability to read letter at certain distance; dynamic visual acuity (动视力) ability to see objects in relative motion with the eyes , Field of vision (视野)--- including acute or clear vision cone with 3 to 10 degree around the line of sight; fairly clear vision cone with 10 to 12 degree around the line of sight ; peripheral vision(周边视力) is considered a critical factor in the driver’s estimation of speed. Awareness of variability --- percentile values are often used in the traffic engineering design in order to safely accommodate the vast majority of users. Distribution of driver’s characteristics is the best way to replicate the real world (e.g. simulation modeling). Reaction time and visual acuity are discussed for group of drivers only. Walking speed ---- a very important element in traffic design for road users, particularly in signal timing (信号配时)at intersections Table 2.1: 50th Percentile Walking Speeds for Pedestrians of Various Ages 50th Percentile Walking Speed(ft/s) Age(years) 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 20-29 30-39 40-49 50-59 60+ Males 2.8 3.5 4.1 4.6 4.8 5.0 5.0 5.1 5.5 5.2 5.8 5.3 5.1 5.6 5.2 5.2 4.9 5.7 5.4 5.1 4.9 4.1 Females 3.4 3.4 4.1 4.5 5.0 5.0 5.3 5.4 5.4 5.2 5.7 5.6 5.3 5.3 5.4 5.4 N/A 5.4 5.4 5.3 5.0 4.1

(Compiled from Eubanks, J. and Hill, P., Pedestrian Accident Reconstruction and

Litigation, 2nd Edition, Lawyers & Judges Publishing Co., Tucson,AZ, 1999.)

Gap acceptance (可接受间隙)--- very important element related to capacity analysis and safety and will be discussed in detail later Comprehension of control information --- it is utmost important for pedestrians to understand and adherence to control devices while crossing streets Impacts of drugs and alcohol --The past studies show that the effect of drugs and alcohol has impaired driver’s ability to react, make correct judgment and execute necessary action. Enforcement and education are considered major weapons in combating DWI (Driving While Intoxicated 醉酒驾车) and DUI (Driving Under Influence 酒后驾车), as there is not a great deal that can be done through design or control to address the issue. Impacts of aging of drivers --As life expectancy continues to rise, the number of older drivers has risen dramatically. It is keenly important for traffic engineers to understand how aging affects driver needs and limitations and how these should impact design and control decision. It should be noted that the most important human factors that influence driving is the personality and psychology of the driver. However, this is not easily quantified and it is difficult to consider in design. For details about road user characteristics, read “Transportation Psychology” 2.2 Vehicle Characteristics: Design vehicle (standard vehicle) --- selection of design vehicles can have a profound effect on the use of roadway facilities such as turning radii, turning pocket length, U-turn from left-turn-bay, and design of parking lot. (see Table 3-1, Table 3-2, Figure 3-6 next three pages) For the specification of vehicles of China, see the Table 2-7 on page 36, 37 of supplementary text 1. Acceleration performance of vehicles --- determination of the length of acceleration lane when merging, dual objectives of greater power and improved fuel efficiency (after 1973-74 fuel crisis and emission requirement of EPA=Environment Protection Agent) Braking performance --- factors affecting braking performance including type and condition of the tires, roadway surface condition

(wet or dry), and grade. The equation of braking distance is given as follows.
db = S i2 − S 2 f 254( F ± G )
(2-1)

where: db = braking distance (meters) Si = initial vehicle speed (km/h) Sf = final vehicle speed (km/h) F = coefficient of forward friction between tires and roadway surface G = grade, expressed as a decimal

For designation of grades, “+” is used for upgrade (uphill) and the “—“is used for downgrades (downhill). It should be noted that AASHTO Green book observes that friction factors F should be based on wet pavements rather than dry. Safe stopping sight distance ( 停 车 视 距 ) --- relating to perception-reaction time and braking performance of vehicle;
d = 0.278S i t + S i2 − S 2 f 254( F ± G )
(2-2)

where d is in meters and Si and Sf are in km/h, t stands for PRT in second.

Application of SSD: Highway design: alignment (vertical or horizontal) must allow drivers to see a distance at least equal to the minimum safe stopping distance. Clearance intervals ( 清 空 时 间 ) at signalized intersections: Yellow plus All Red should be long enough to allow vehicles to stop safely at intersections. (dilemma zone --- an area close to an intersection in which a vehicle can neither stop safely before the intersection nor clear the intersection without speeding before the red signal come on.) Sign placement: e.g. sign before toll plaza Accident investigations: using skid marks on pavement to estimate vehicle speeds Decision sight distance (决策视距)--- 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. (see Table 2.6 page 35) Climbing capability ---

High rear end crash on the two-lane highways; Big difference in speed between truck and passenger car when grade is steep Climbing lane 2.3 Geometric Characteristics of Roadways: Horizontal alignment (平曲线) refers to a plan view of the highway --Vertical alignment (竖曲线)--- refers to the design of the facility in profile view Cross section (横断面)--- refers to a cut across the plane of the highway All these contents are contained in Green Book and will be studied in geometric design course. The current geometric design of roadways that is different from previous practice is the change from vehicle-oriented to human-centered concept. 2.3.1 Horizontal alignment of highways Geometric characteristics of horizontal curves Radius and degree of curvature Review of trigonometric functions Critical characteristics of circular horizontal curves Length of tangent Length of long chord Superelevation of horizontal curves Spiral transition curves Length of spiral Angle of deflection for the spiral Length of tangent distance Sight distance on horizontal curves Compound horizontal curves Reverse horizontal curves 2.3.2 Vertical alignment of highways Grades --- defined as longitudinal slope Geometric characteristics of vertical curves --- crest versus sag Sight distance on vertical curves Other minimum controls on length of vertical curves Some design guidelines for vertical curves 2.3.3 Cross-section elements of highways Travel lanes and pavement Shoulders --- defined by AASHTO as portion of roadway contiguous

with the traveled way that accommodates stopped vehicles, emergency use, and lateral support of sub-base, base, and surface courses of the roadway structure. Side-slopes for cuts and embankments Guardrail --- roadside guardrail is provided to prevent vehicles from entering a cross-slope than 4:1, or from colliding with roadside objects such as trees, culverts, lighting standards, sign posts. Median guardrail is primarily provided to prevent vehicles from encroaching into the opposing lane of traffic

2.4 Traffic Control Devices: Definition --- the media traffic engineers use to communicate with drivers Objectives --- (1) communicate with road users in a manner that information delivered to users has a high level of expectance and comfort (2) consistent in providing standardization of sign, markings and signal for drivers to avoid confusing MUTCD --- Manual on Uniform Traffic Control Devices

2.4.1

Traffic Signs --- to regulate, warn or guide trafficRegulatory signs (法令标志) --- convey information related to specific traffic regulations such as right-of-way (通行权), speed limit, lane usage, parking. Warning signs (警告标志)--- inform drivers about upcoming hazards that they might not see Guide signs (指路标志) provide information on routes, destinations --and services that drivers may look for Traffic Markings --- to channelize traffic by painting or delineating on the surface of road Longitudinal markings (纵向标线)--- refer to those markings placed parallel to the direction of travel such as: centerline, lane lines, and pavement edge lines Transverse markings (横向标线) --- refer to any markings with a component that cuts across a portion or all of the traveled way such as STOP lines, crosswalk markings, parking space markings, word and symbol markings Object markers and delineators (物标和反光灯)--- object markers are used to denote obstructions; delineators are reflective devices mounted on the side of roadways to help denote its alignment Signals --- to direct and warn traffic Traffic control signals --Pedestrian signals --Signal at freeway entrance --Signal for draw bridges (开启桥) --Lane-use control signal (车道灯) --Flashing beacon (闪耀灯) --Detailed of traffic control devices will be discussed in chapter 7. The above section refers to Chapter 2 and Chapter 3 of the “Traffic Engineering” on pages 17—70.

2.4.2

2.4.3

本章要点: 1.驾驶员反应时间包含哪几个过程 2.动视力与静视力的关系 3.为什么要确定标准车 4.刹车距离和安全停车视距的计算及其应用 5.现代几何设计与过去的主要区别 6.交通控制设施设计的目标是什么,交通控制设施包含哪几方面


				
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