Highway Eng.
Turning Paths & Sight Distance
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Design Vehicle, Turning Paths and Sight Distance 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 distance B – Passing sight distance The information listed in this lecture is mainly taken from the Policy on Geometric Design of Highways and Streets (AASHTO, 2011), Iraqi Highway Design Manual (SORB, 2005) and Traffic and Highway Engineering 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 engineer’s measure of existing traffic volume. vo lume. Lecture 02
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Dr. Firas Asad
Highway Eng.
Turning Paths & Sight Distance
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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 safety studies. 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 Tr affic Projection Factor (TPF): F. ADT = C . ADT * TPF TPF
= ( 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 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. DHV = F. ADT * K
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 Lecture 02
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Dr. Firas Asad
Highway Eng.
Turning Paths & Sight Distance
14 –15
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 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 sightdistance requirements. AASHTO has suggested the following guidelines for selecting a design vehicle: - For a parking lot or series of parking lots, a passenger passenger car may be used. used. - For intersections on residential streets, streets, a single-unit truck could 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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Highway Eng.
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Highway Eng.
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Dr. Firas Asad
Highway Eng.
Turning Paths & Sight Distance
14 –15
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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Dr. Firas Asad
Highway Eng.
Turning Paths & Sight Distance
14 –15
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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Dr. Firas Asad
Highway Eng.
Turning Paths & Sight Distance
14 –15
3- Sight Distance A driver’s 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 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). Lecture 02
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d 2
V
=
Turning Paths & Sight Distance
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2
a ± G 9.81
254
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 driver can 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 the overtaken vehicle
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Highway Eng. -
Turning Paths & Sight Distance
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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:
d 1
at = 0.278 * t i v − m + i 2
Where: ti: time of initial manoeuvre, 3.7-4.3sec;
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Dr. Firas Asad
Highway Eng.
Turning Paths & Sight Distance
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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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Dr. Firas Asad
Highway Eng.
Turning Paths & Sight Distance
14 –15
C - Sight distance at horizontal curves: curves : 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:
m
28.65S = R 1 − cos( ) ; R
m is also called horizontal sightline offset HSO.
Example: Horizontal curve having having a radius of 300m forms part part of two-lane highway 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: 2
2
SSD = 0.278 V t + 0.039 (V /a) = 0.278 (70)(2.5) + 0.039 ((70) / 3.4) = 105 m
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Highway Eng.
Turning Paths & Sight Distance
14 –15
Then compute m (HSO)
m
28.65S 28.65(105) = R 1 − cos( ) = 3001 − cos( ) = 4.58 m. 300 R
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