BASIC CHARACTERISTIC ROAD SYSTEMS ( Part 1 )

Highway Engineering C3010 / UNIT 11

BASIC CHARACTERISTICS OF A ROAD SYSTEM

( Part 1)

OBJECTIVES

General Objective

To understand the basic characteristic of a road system.

Specific Objectives

At the end of the unit you should be able to :-

· state the characteristics that will influence the system.

· identify the related factors of the system.

· describe the importance characteristics of drivers.

11.0 INTRODUCTION

A Road Design includes Geometry and Pavement Design. These designs are require in providing information on traffic flow. The highway engineer must design for a wide range of vehicle operating characteristics and allow for great differences in driver and pedestrian characteristics. Most highway facilities must be designed to accommodate the smallest subcompact automobile as well as the largest tractor-trailer truck. In may instances, the design must also accommodate motorcycle and bicycle users and pedestrians. It should be remembered that within each class of users there is great variability; differences in vehicle sizes, weights, and operating characteristic ability to comprehend and react to highway features and traffic events.

11.1 ROAD SYSTEM CHARACTERISTIC

There are several characteristics in the road system.

11.1.1 DRIVERS CHARACTERISTICS

Drivers in Malaysia are licensed to drive under laws. The differences about age, skill and experience also can produce some impact to the traffic flow. It is important that highway engineers keep in mind that street and highway facilities must be designed to accommodate driver with a wide range of ages and skills, the young and the elderly and the novice as well as the experienced professional.

11.1.2 VEHICLE CHARACTERISTICS

Vehicle characteristics also influence the traffic flow in any road systems. Vehicles are used for transportation on the road or highway. All types of vehicles have shapes, characteristics and own usage.

11.1.3 TRAFFIC CHARACTERISTIC

Traffic characteristic are the most important characteristic in the traffic flow for any road systems.

11.2 IMPORTANT CHARACTERISTIC OF DRIVERS

The driver characteristics must be identified initially before a certain geometry design and other related road design is implemented or proceed. The drivers are also considered as the road user. There are several factors that influence driver’s, such as physical, environment and psychology factors.

11.3 INFLUENTIAL FACTORS.

There are some factors that would influence the characteristics of a driver.

11.3.1 PHYSICAL FACTORS

The driver’s decisions and actions depend principally on information received through the senses. This information comes to the driver through the eyes, ears, and the sensory nerve ending in the muscles, tendon, joints, skin, and organ. In general the order of importance of the senses used by drivers are;

1. Visual ( sight ).

2. Kinesthetic ( movement ).

3. Vestibular ( equilibrium ).

4. Auditory ( hearing ).

Driver perception-reaction time is defined as the interval between seeing, feeling or hearing a traffic or highway situation and making initial response to what has been perceived. People generally react more quickly to very strong stimuli than to weak one.

11.3.2 ENVIRONMENTAL FACTORS

Weather and road condition also influence the characteristics of drivers. Most of the drivers will take an extra cautious when the road is wet and the road surface is damaged. This condition will endanger the safety of driver.

11.3.3 PSHYCOLOGICAL FACTORS.

New drivers become nervous compare to the experienced drives whenever they are driving in a crowded traffic area. They feel safer driving through a straight road would speed more.

11.4 VEHICLE CHARACTERISTICS

The standard of road design is influenced by the characteristic of vehicles that used the road. The sizes of motor vehicle influence clearances for bridges, tunnel, and grade separation structure and the geometric characteristic of streets, roads, and parking facilities.

11.4.1 RESISTANT

A vehicle’s motion tends to be retarded by at least five types of resistance:

1. Inertia resistance.

2. Grade resistance.

3. Rolling resistance.

4. Curve resistance.

5. Air resistance.

11.4.1.1 Inertia resistance.

It will be recalled from the study of physics that inertia is the tendency of a body to resist acceleration; the tendency to remain at rest or to remain in motion in a straight line unless acted upon by some forces. The forces, Fi, required to overcome a vehicle’s inertia is described by the familiar relationship

Fi = ma = W/g a, kg

Where

m = vehicle mass

a = acceleration ( m/s2)

W = vehicle weight ( kg )

g = acceleration force due to gravity ( 9.81 m/s2)

When acting to move a vehicle forward, this force is positive. When the force is slowing or stopping a vehicle, it and the corresponding acceleration (deceleration) are negative.

11.4.1.2 Grade resistance.

Grade resistance is the component of the vehicle weight acting down a frictionless inclined surface. As figure illustrates below, the magnitude of the forces is directly proportional to the gradient. By similar triangles;

G /100 = Fg/Fn

Fg = FnG/100 = W cos q G /100

Where

G = gradient ( percent)

W = vehicle weight ( kg )

Fg = grade resistance ( kg )

Fn = normal forces ( kg )

For even the steepest highway gradients encountered in practice,

cos q » 1.0.

For practical purpose, the gradient resistance is

Fg = WG/100

11.4.1.3 Rolling resistance.

A vehicle does not operate on a smooth friction surface. There is resistance to motion as the tires roll over irregularities in the surface and push through mud, sand or gravel. This resistance, termed rolling resistance, includes that caused by the flexing of the tires and the internal friction of the moving part of the vehicle.

11.4.1.4 Curve resistance

As was stated earlier, once a vehicle is set in motion, it tends to remain in motion in a straight line unless acted on by some force. The forces changing the direction of a vehicle are imparted through the front wheels. Components of these forces tend to impede a vehicle’s forward motion. Curve resistance then is the force required to cause a vehicle to move along a curve path. It is a function of the radius or degree of curvature and the vehicle speed.

11.4.1.5 Air resistance.

Air resistance includes the force required to move air from a vehicle’s pathway as well as the friction effects of air along its top, sides, and undercarriage. It is a function of the frontal cross-sectional area of the vehicle and the square of the vehicle speed.

11.5 VEHICLE OPERATING CHARACTERISTIC

There are various vehicle-operating aspects that considered for geometry design purpose. The vehicle-operating characteristics that influence are road geometry design, curves radius, acceleration and braking.

11.6 ACCELERATION AND DECELERATION

A motor vehicle move according to fundamental laws of motion; relationships between distance, time, velocity and uniform acceleration are given by the following equation:

Vf = vo + at

D = vot + ½ at2

V2f = v2o + 2ad

Where

vf = final velocity ( m/s )

vo = initial velocity ( m/s )

a = acceleration or deceleration

t = time ( sec )

d = distance ( m ).

Maximum acceleration rates vary with the size of the vehicle and its operating speed. Vehicles are capable of greatest acceleration at lowest speeds. From a standing start to a speed of 15 mph, maximum acceleration value range from about 2 mph/sec for tactor-semitrailer trucks up to about 10 mph/sec for large cars. For a speed change of 0 to 30 mph, typical maximum acceleration are for tractor semitrailer truck – 4.57 m/s2, for large car – 3.14 m/s2 and for small high performance sports car – 4.33 m/s2.

Without braking, a vehicle will decelerate when the driver release the accelerator due to the drag of the engine, air resistance, grade resistance and so forth. A passenger car operating in the range of 50 to 60 mph will decelerate about 0.91 m/s2 without braking; in the range of 20 to 30 mph, and automobile will decelerate about 0.46 m/s2.

Under normal braking conditions, the levels of deceleration developed usually do not reach the limit of vehicle’s braking capability nor that of the pavement-tire interface. The deceleration may be limited by either the condition of the brakes or that of the tires and roadway surface. In panic situations, most drivers tend to apply the brakes to the extent that a locked wheel skid develops, even though a greater frictional force is developed before skidding occurs.

TEST YOUR UNDERSTANDING BEFORE YOU CONTINUE WITH THE NEXT INPUT

1. The motion of the vehicle tends to be retarded by at least five types of resistance. State the type of vehicles resistance.

2. Describe the grade resistance.

3. This formula is to determine acceleration or deceleration velocity. Find the meaning of this formula,

V2f = v2o + 2ad

Where;

V2f = ______

v2o = ______

a = ______

d = ______

1. A vehicle’s motion tends to be retarded by at least five types of resistance:

i. Inertia resistance.

ii. Grade resistance.

iii. Rolling resistance.

iv. Curve resistance.

v. Air resistance.

2. Grade resistance is the component of the vehicle weight acting down a frictionless inclined surface. As figure illustrates below, the magnitude of the forces is directly proportional to the gradient. By similar triangles;

G /100 = Fg/Fn

Fg = FnG/100 = W cos q G /100

Where

G = gradient ( percent)

W = vehicle weight ( kg )

Fg = grade resistance ( kg )

Fn = normal forces ( kg )

For even the steepest highway gradients encountered in practice,

cos q » 1.0.

For practical purpose, the gradient resistance is

Fg = WG/100

3. V2f = v2o + 2ad

Where

vf = final velocity ( m/s )

vo = initial velocity ( m/s )

a = acceleration or deceleration

t = time ( sec )

d = distance ( m ).

1. Describe briefly the factors that would influence the characteristics of a driver.

2. Describe the following aspects: -

a. Curve resistance.

b. Air resistance

1. There are some factors that would influence the characteristics of a driver.

PHYSICAL FACTORS

a. Visual ( sight ).

b. Kinesthetic ( movement ).

c. Vestibular ( equilibrium ).

d. Auditory ( hearing ).

ENVIRONMENTAL FACTORS

PSHYCOLOGICAL FACTORS.

New drivers become nervous compare to the experienced drives whenever they are driving in a crowded traffic area. They feel safer driving through a straight road would speed more.

2. Curve resistance

Air resistance.