lec 02 highway engineering - design parameters turning paths and sight distance

7/25/2019 Lec 02 Highway Engineering - Design Parameters Turning Paths and Sight Distance

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Lecture 02 17

Highway Eng. Turning Paths & Sight Distance 14 15

Dr. Firas Asad

In this lecture;---------------------1- Highway Design Parameters.

A- Traffic volume.B- Design vehicle.C- Design speed.

2- Minimum Turning Path.3- Sight Distance

A Stopping sight distanceB Passing sight distance

Design Vehicle, Turning Paths and Sight Distance

The information listed in this lecture is mainly taken from the Policy on GeometricDesign of Highways and Streets (AASHTO, 2011), Iraqi Highway Design Manual(SORB, 2005) and Traffic and Highway Engineering (Garber and Hoel, 2009).

1- Highway Design Parameters

Following are some of the key design parameters (controls) used in highway design.

A- Traffic volume

Traffic volume is the number of vehicles and/or pedestrians that pass a point on a

highway facility during a specified time period. This time period varies from as little

as 15 minutes to as much as a year depending on the anticipated use of the data.

In traffic engineering studies there are many volumes such as weekly volume, daily

volume and peak hour volume. Traffic volumes of a day or an hour can vary greatly,

depending on the different days of the week or different time period of a day.

Average Daily Traffic (ADT): ADT is the volume that results from dividing a traffic

count obtained during a given time period by the number of days in that time

period. For example, given a traffic count of 52,800 vehicles that was taken over a

continuous period of 30 days, the ADT for this count equals 1,760 vehicles

(52,800/30). The ADT is the traffic engineers measure of existing traffic volume.


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Annual Average Daily Traffic (AADT) : Another commonly used measure of traffic

volume is the AADT, which is determined by dividing the total yearly traffic volume

(in both direction) by 365. It is usually used in highway classification and some safetystudies.

Projection of Future Traffic Demands

Future average daily traffic (F. ADT) volume can be forecasted based on the current

average daily volume (C . ADT) and the Traffic Projection Factor (TPF):

F. ADT = C . ADT * TPFTPF = ( 1 + r) x+n

where

r = annual rate of traffic growth, (0.02- 0.12).x = number of construction years.n = design period or life, (15-30 years).

Design Hour Volume (30 HV)

The design hourly volume (DHV) is a future

DHV = F. ADT * K

peak hourly volume used for design.

Design Hour Volume is the highest hourly volume that is only exceeded by 29 hourly

volumes during a designated year. The DHV is a two-way traffic volume that is

determined by multiplying the F.ADT by a design hour factor called the K-factor.

Values for K typically range from (0.08 to 0.12) for urban facilities and (0.12 to 0.18)

for rural facilities.

In order to compute the directional design hourly volume DDHV which is the DHV in

the peak direction, the directional distribution (DD) factor should be used. The DD is

the proportion of the peak-hour traffic travelling in the peak direction (expressed as

decimal).

DDHV = F.ADT * K * D


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B- Design vehicle

The size of the design vehicle for a highway is an important factor in the

determination of design standards for several physical components of the highway.

These include lane width, shoulder width, length and width of parking bays, and

lengths of vertical curves. The axle weights of the vehicles are important when

pavement depths and maximum grades are being determined.

Passenger cars, buses, trucks and recreational vehicles are the four standard classes

of vehicles on roads and highways. Hence, it is essential that design criteria take into

account the characteristics of these different types of vehicles.

Design vehicle

AASHTO has suggested the following guidelines for selecting a design vehicle:

is that vehicle whose characteristics include those of nearly all

vehicles expected to use the highway. The characteristics of the design vehicle are

used to determine criteria for geometric design, intersection design, and sight-

distance requirements.

- For a parking lot or series of parking lots, a passenger car may be used.

- For intersections on residential streets, a single-unit truck could be considered.

- For the design of intersections of state highways and city streets that serve bus

traffic but with relatively few large trucks, a city transit bus may be used.

- For the design of intersections of low volume highways (AADT of 400 or less), a

large school or conventional bus may be used.

- For the design of intersections high volume highways and for intersection of

freeway ramps with arterial highways, the WB-20 (WB-65 or 67) truck may be

used.


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C- Design speed

As studied in Traffic Engineering subject last year, speed is the rate of movement

with time, and there are the several types of vehicle speeds. Following are the most

common in speed traffic studies:

- Spot speed- Running speed- Travel speed- Design speed

Design speed is the maximum safe speed that can be maintained over a specified

section of the roads or highways when conditions are so favourable that the design

features of the highway govern. The design speed is depend the highway

classification, traffic composition, urban and rural areas, and topography.

Except for local streets, every effort should be made to use as high a design speed as

practical to attain a desired degree of safety, mobility, and efficiency within the

constraints of environmental quality, economics, aesthetics, and social or political

impacts.


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2- Minimum Turning Path

In carrying out the design of any intersection, the minimum turning radius for the

selected design vehicle travelling at a speed of 10 mph should be provided.Minimum turning radii at low speeds (10 mi/h or less) are dependent mainly on the

size of the vehicle. The turning-radii requirements for single-unit (SU) truck and the

WB-20 (WB-65 and WB-67) design vehicles are given in Figures below respectively.

The turning-radii requirements for other vehicles can be found in AASHTO s Policy

on Geometric Design of Highways and Streets.


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3- Sight Distance

A drivers ability to see ahead is of the highest importance in the safe and efficient

operation of a vehicle on a highway. Sight distance is the length of the roadway

ahead that is visible to the driver. Four aspects of sight distance are important to be

considered in highway geometric design:

(1) the sight distances needed for stopping, which are applicable on all highways;

(2) the sight distances needed for the passing of overtaken vehicles, applicable only

on two-lane two-way highways;(3) the sight distances needed for decisions at complex locations; and

(4) the criteria for measuring these sight distances for use in design.

A- Stopping sight distance - SSD

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 on the bath.

SSD = d1 + d2

d1 - the distance traversed by the vehicle from the instance the driver sights an

object necessitating a stop to the instant the brakes are applied; and

d2 - The distance needed to stop the vehicle from the instant of brake application

begins.

d1 = 0.278 V * t ,

Where:d1: the brake reaction distance, m;

V: design speed, km/h;t : the brake reaction time, sec (typically 2.5 s).


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=G

a

V d

81.9254

2

2

Where;

d2: the braking distance, m;V: design speed, km/h;a: deceleration rate, m/s2 ( a = 3.4m/s2 comfortable for most drivers)

G: is the percent of longitudinal road grade, in decimal. The SSD needed on

upgrades is shorter than on level roadways; those on downgrade are longer.

B- Passing sight distance - PSD

Passing sight distance for use in design should be determined on the basis of the

length needed to complete normal passing manoeuvres in which the passing drivercan determine that there are no potentially conflicting vehicles ahead before

beginning the manoeuvre. The following assumptions are made concerning driver

behaviour in passing manoeuvres:

- the overtaken vehicle travels in uniform speed;

- the passing vehicle has reduced speed and trails the overtaken vehicle as it

enters a passing section;

- when the passing section is reached, the passing driver needs a shorter period

of time to perceive the clear passing section and to react to start the

manoeuvre;

- the passing vehicle accelerates during the manoeuvre, and its average speed

during the occupancy of the left lane is 15km/h higher than that of theovertaken vehicle


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- when the passing vehicle returns to its lane, there is suitable clearance length

between it and an incoming vehicle in the other lane

As shown in the figure above, the minimum passing sight distance for two-lane

highway is determined as the sum of the following four distances:

SSD = d1 + d2 + d3 + d 4

d1: distance traversed during perception and reaction time and during the initial

acceleration to the point of encroachment on the left lane. d1 called initial

manoeuvre distance, can be calculated by:

+=

2

atmvt*278.0d i

i1

Where:

ti: time of initial manoeuvre, 3.7-4.3sec;


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a: average acceleration, km/h/sec (about 2.4 km/h/sec);

v: average speed of passing vehicle, km/h;

m: difference in speed of passed and passing vehicle, 15 km/h .

d2: distance while passing vehicle occupying left lane. Can be calculated by:

d2 = 0.278 v * t2

Where:

t2: time passing vehicle occupies the left lane, sec (9.3-10.4 sec);v: average speed of passing vehicle, km/h;

d3: distance between the passing vehicle at the end of its manoeuvre and the

opposing vehicle, called clearance length. It is found vary from 30-75m.

d3 = (30 75) m

d4: distance traversed by an opposing vehicle for two-thirds of the time the passing

vehicle occupies the left lane,

d4= 2/3 d2


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If a vehicle is located at point A on

the curve and the object is at

point B, the line of sight is the

length of chord AB. The horizontal

distance traversed by the vehicle

when moving from point A to

point B is the length of arc AB.

The radius of curvature R, the

setback distance m, and the

stopping sight distance s is given

by the following metric formula:

C - Sight distance at horizontal curves

:

= )65.28cos(1 RS

Rm ; m is also called horizontal sightline offset HSO.

Example: Horizontal curve having a radius of 300m forms part of two-lane highway

that has a posted speed limit of 70km/h. if the highway is flat at this section,

determine the minimum distance a large billboard can be placed from the center

line of the inside lane of the curve, without reducing the required SSD. Assume

perception- reaction time of 2.5 sec.

Solution :

Firstly, we have to determine the required SSD:

SSD = 0.278 V t + 0.039 (V 2/a) = 0.278 (70)(2.5) + 0.039 ((70) 2/ 3.4) = 105 m


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Then compute m (HSO)

= )65.28

cos(1 RS

Rm = )300)105(65.28

cos(1300 = 4.58 m.

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lec 02 highway engineering - design parameters turning paths and sight distance

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