road user and the vehicle

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ROAD USER AND THE VEHICLE

10.1 IntroductionObjecttves

10.2 Human Factors Governing Road Use Behaviour

10.3 Power Performance of Vehicles

10.4 Other Vehiclei Characteristics

10.5 Decelerationof Vehicles during Coasting

10.6 Characteristics of Slow Moving Vehicles

10.7 Illustrative Problems

10.8 Summary

10.9 Key Words

10.10 Answers to SAQs

14.1 INTRODUCTION

traffic involves a close intersection among vehicles, the road user and the road.e railway transport, there is no fixed or guided path for the vehicles, which canat any desired speed and occupy any position on the pavement. A fullstanding of the characteristics of the vehicles that move on roads is a basicement before measures to deal with traffic can be thought o f . The road user is

entity whose behaviour influences significantly the traffic manoeuvres and trafficRoad users can be pedestrians, drivers or cyclists. In this unit, you will learn theistics of vehicles and the road users.

afBa tudying this unit, you should be able to

I nderstand the human factors which govern the behaviour of road Grcrs,

tknow how the power developed by the engine of a road vehicle is utilised inovercoming various types of resistance,1 know how a vehicle decelerates when allowed to coast in neutral gear, and

understand the characteristics of slow moving vehicles, which are also present inconsiderable number on Indian roads.

10jk HUMAN FACTORS GOVERNING ROAD USE.

BEHAVIOURAs ou know, vehicles are driven by human beings. Cycles, cycle rickshaws, handcartsand nilnal drawn vehicles also involve human behavioui. Pedestrians are another groupof r ad users whose behaviour also influences the road traffic.t

lowing human traits are important to you as a traffic engineer:

Vision

Hearing

) Psych.ologica1 characteristics in perceiving, analysing andreacting to road traffic situations

) Speedofwalking



Traffic Engineering Drivers an d pedestrians m ust be able to see clearly the road and traffic con d~t ion so

enable them to react to the situation. Vision is thus a very impc>rtrtanr trait of the road userhaving a bearing o n traffic safety.

The zone of clear vision is li~llitcd .The zone of clear vision is known as the zone of

acute vision or visual acuity. This zone is formed by a cone who se angle is around 3"

about the centre of the retina (Figure 10.1). However, vision is still satisfactc>ry or

angles up to 1 0 " or 12".

Visual Acuity 10-12"atisfactory Vision)

(Very C lear Vision)

Figure 10.1: Visual Acuity and Peripheral V i s i n ~ ~

Peripheral vision is the visual field within which the two eyes together ca n see an objecl,

but without clear details and colour. The angle of peripheral vision is about 160" in the

horizontal direction and 115' in the vertical direction under static conditions. The angledrops down as the speed of the driver increases.

Hearing is an extrem ely important aid to a road user. He can avert accidents when hehears the sound of other vehicles or horn of vehicles.

When a road user perceives a danger on the road he immediately reacts to it. Th e speedwith which he reacts depends on his psychological makeup. Th e reaction involves thefollowing four processes:

Perception Process of perceiving the sensation received throughthe eyes, ears, nervous system and brain.

Intellection Process of identifying the stimuli crireria by

perception and ideas.Emotion The personal trait of the individual governing his

decision making process.

Volitioll The will to react to a situation

The above four are commonly known as PIEV. The PIEV time is around 2.5 sec. Thisimplics that in a normal person, a time of 2.5 sec elapses before he perceives a danger

and reacts to it.

The speed at which a person walks is an important input in designing pedestrianfacilities. The speed varies from 0.75 to 1.8m per sec. For design of pedestrian signals, aspeed of 1.2 m per sec is norlnally considered.



I110,311 POWER PERFORMANCE OF VEHICLES

Koad U ~ e r nd the Vehicle

developed by the engine of 1ndc)r vehicle is used for overcoming various

Rolling resistanceP/ l ~ i resistance

brade resistance

resistance is the frictional force that develops between the rolling tyres and the

' It depends upon the smoothness of the surface and the speed of motion.

The rclling resistance (Figure 10.2) is given by:bPf= mgf

here,

Pf= Rolling resistance, N

rn = mass of vehicle, kg

= acceleration due to gravity, m/sec2

,f= coefficient of rolling resistance

fficient of rolling resistance varies from 0.01 for a smooth asphaltic surface to

a rough earthen road.

Figure 10.2: Forces Acting on a Vehicle

is the resistance to motion offered by the air. The frontal area.& the

the motion. The resistance increases with the square of the speed of the

formula holds good:

whe e,I a = Air resistance, N

A = frontal area, m2v = speed, dsec.

C ,= coefficient of air resistance, kg/m3

Grade r ista~lces the force spent in overcoming the force of gravity. It is given by the

express' n:

Pi = rngi

where,

Grade resistance,N

= mass, kg

= acceleration due to gravity, ndsec2.

= grade angle, expressed as a fraction



Traffic Ellgineering The e ngine has to spend additional energy whenever it accelerates or decelerates. If the

vehicle has to start from a stopped condition and if it atta ins the desired speed afteraccelerating, the force required to overcom e the inertia is given by:

P, = mass x accelcration, N

= ma

where,m = mass,kg

a=

acceleration, m/sec2.If the vehicle decelera tes, the above force will be negative.

The force developed by the engine has to be ultimately transferred to the driving wheelConsiderable loss may take place in this process. The loss riiay be lo lhe extent of 15 to25 per cent.The d riving axle torque is given by the expression:

where,T, = driving axle torqueK = efficiency of transmission, which can be i n the range of 0.75 to 0.85.

Gt= transmission gear ratio.G, = rear axle gear ratio

The tractive force Pi,,mparted by the deriving axle torque at the interface between thetyre and the road is given by:

driving axle torquePP=

radius of the rolling driv e

The radius of the rolling drive is slightly smaller than the radius of the tyre because of tflattening of the tyre. The radius of the rolling drive, r , can be in the range of 0.93 to

0.95 of the radius of the tyre, r, (Figure 10.3).

When the vehicle moves at a speed, the power output corresponding to the driven speedis given by:

Power Output = Pp v

where,Pp = tractive force , N

v = speed, m lsec

Figure 10.3: Tyre-Road Surface Interaction



where, 1Y V = speed in krdh

:..POW r output of engine= Pp.vf

Pp 0.377.rw.n, in watts

G, .G,zKI Engine power in wattsEngin horsepower (metric) =

735

Pp,the tractive force = Rolling resistance + Air resistance + Grade ResistanceI + inertia forces due to acceleration and deceleration.

engineer has to have knowledge of some other characteristics of vehicles. Table

gives the dimensions of typical Indian vehicles.

10.41

Rt);tc! I ; E ~ I -ud the V e l l i r l ~ ,

OTHER VEHICLES' CHARACTERISTICS

I, Table 10.1: Dimensions of Indian Vehicles

The ac eleration of typical vehicles is:44

Vehicle

Ambassador

Maruti

Truck

Car : 3 m/sec2.

Truck: 0.5 m /sec2

deceleration rate for cars is 1.5-2.0m/sec2.

are applied to vehicles, friction is developed between the tyre and the roa:.

'?ype

Car

Ca r

Bu s ~I

Breadth

(m )

1.68

1.40

2.5

2.5

Length

(m )

4.31

3.60

7 OO

9.00

Height

(m )

1.60

1SO

3.10

Mass Fro nh l Area

(kg) (m2)

1365 2.15

880 1.54

6120 (Empty) 5.37

3.20 7500 (Empty) 5.37



Traffic Engineering where,

v .: peed, mlseca = deceleration, mn/sec2d = distance travelled, m

The coefficient of friction mobilised is given by:

where,a = deceleration, m/sec2g = deceleration due to gravity, m/sec2

Th e coefficient of friction varies from 0.3 for wet smooth surfaces to 1.0 for dry roughsurfaces.

10.5 DECELERATION OF VEHICLES DURING COASTING

When a vehicle is travelling at a speed of v m/sec, if its engine is sudden ly put off, thevehicle coasts to a stop. The forces that cause it to come to a stop are the rollingfrictional resistance, air resistance and grade resistance.

The plus sign for Pi comes into effect while negotiating upward grades and minus signco'mes into effec t while negotiating negative grades.

Since the veh icle decelerates when the engine is shut off,

Pp= 0

P, = Negative in value

2. ma =m.g. + Ca.A.v fmgi

10.6 CHARACTERISTICS OF SLOW MOVING VEHICLES

Slow moving vehicles include cycles, cycle rickshaws, bullock carts and horse drawnvehicles. Som e of the characteristics of these vehicles are given in Table 1 0.2.

Table 10.2: Characteristics of Slow-Moving Vehicle

Vehicle Type

Cycle

Cycle Rickshaw

Bullock Cart

Horse Cart

Breadth

(m)

0.60

1.10

1SO

1.40

Length

(4

1.90

2.60

5.90

4.10

Mean Speed

(kmlh)

15

8

4

12



Road User and the Vehicle

a) What is visual acuity '?

b) What is the angle of most clear vision'?

c) What is the angle of satisfactory vision'?

d) What is the angle of peripheral vision in the horizontal direction and in thevertical direction'?

e) What PIEV? What is the value of PIEV time'?What is the speed of w d k of a person'?

g) What are the forces a vehicle has to overcome while in motion'?

h) How is rolling resistance calculated?

i) What is the value of coeff icient of rolling friction'?

j) How is air resistance calculated'?

k) How is grade resistance calculated?

1) How are the inertia forces due to acceleration and the decelerationcalculated?

m) How m uch is the usual loss in transmission in the vehicle?

n) What is the width of a bus and a truck?o) What is the height of a truck?

p) When brakes are applied to a vehicle what is the coefficient of frictiondeveloped?

q) When a vehicle is in m otion, and its engine is suddenly shut off, give theforce equation.

r) What is the average speed of a cycle'?

s) What is the avera ge speed of a bullock cart?

10.7 ILLUSTRATIVE PROBLEMS

is required to accelerate at a rate of 2 m ,sec2from a speed of

adient is 4 per cent, upward. The coeff icient of rollingis 0.02. The frontal area is 5.30 sq. m. The truck tyre has a radius of 0.5 m.

condition, the tyre radius can be taken to be 0.95 of its true radius. Theair resistance is 0.48 kg/m3. The transmission efficiency is 0.90. Calc ulate

and the engine RPM. The rear axle ratio is 4 .18: 1 and the



raffic Engineering10+ 20.

Average speed = r, = 15 k d h

15Power Output = P, v =38873.2 x-

.6

Engine horse - power needed =220.4

Transmissi on Efficiency

: n =lOx2.81x4.18

for speed of 10 km/h0.377 x 0.475

= 656 RPM

For a speed of 20 kmih

Problem 2A vehicle travelling at 80km per hour i s brought to a halt by braking. What is thebraking distance, if the coe ftkie nt of friction that develops between the tyres and theroad surface is 0.4 0?

Solution



( Problem3

A car travelling at 1 00 krn per hour is brought to a halt by switching off' the enginc, nobrakes being applied. The coefficient of rolling resistance is 0.02 , the coefficient of airresistance is 0.40 kglm" the frontal area of the car is 2.1mn2and the road has a dow nwardgradent of 1 per cent. Th e mass of the car is 1365 kg. Calculate the distance travelled by

the car.

( Solution

In this case P, = 0, Pi = -11100 and P; is negative.

P , = P I + P a + P i

v 2Distance travelled =-

.d

0.8 SUMMARY

Unit, you have been introduced to the human factors that govern road user

our. These human traits a re vision, hearing, psychological characteristics and

king speed. Typical dimensions of vehicles have been presented. These govern the

ce occupied by a vehicle and its efficient movement. You have also been introducedvarious types of resistance that are to be overcome by the vehicle. These forces arection, air resistance, grade resistance an d inertia forces to he overcome during

cceleration and deceleration. So me losses also take place in transnlission. You haveen made familiar with the equations relating to these. The use of these equations i n

termining the distance travelled by a vehlcle with its engine shut off has also been

is unit has made you understand th e characteristics of the road user and the vehicle.

lp.9 KEY WORDS

Y)sual Acuity The zone of clear vision while on the road. This

zon e is formed by a cone whose angle is around3 degrees abo ve the centre of the retina.

Vision The visual field within which the two eyestogether can s ee an object, but without cleardetails and colour. Th e angle of peripheralvision is about 160degrees in the horizcmtal

direction and 115 degrees in the vertical

Roird Ijser and t l ~ e e l~ ic l r .



Trafic Engineeri~tg . direction.

Perception Th e process of perceiving the sensation receivedthrough the eyes, ears, nervous system and brain.

Intellection The proxess of iden tifying the stimuli criteria byperception and ideas.

Emotion The personal trait of the individual governing his

decision making process.

Volition The will to react to a situation.

PIEV time Total time taken for perception, intellection,elllotion and volition.

Rolling resistance The frictional resistance between the rolling tyresof the vehiclc and the road surface.

Air resistance The resistance to motion offered by the air.

Grade resistance The force spent by the vehicle to overconle theforce of gravity.

Acceleration Rate o f hange of increase of speed.

Deceleration Rate ol' change of decrease of speed.

10.10 ANSWERS TO S A Q s

a) Refer section 10.2

b) 3"

c) 10-12"

d) Horizonla1 direction: 160"

e) Vertical direction: 115"

f) Refer section 10.2

g) PIEV time = 2.5 sec

h) 0.75-1.8 n~per sec.

i ) Refer section 10.3

.i> f= n v , f

k) F= 0.01 to 0.06

1 ) Po = C, A v2

I ) PI= n2.g.i

n) Pi= nl c7

(1) 15 to 25 per cent

p) width = 2.5 nl

q) Height of truck = 3.10 m

r) F=0 .3 t o 1.0

S) P p = 0 = Y + P , + P , - P j

t) 15 km/h

u) 4 hn / h

road user and the vehicle

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