LECTURE 3(a)

Agenda

• Recap and definitions

• LoS and highway capacity

• Factors affecting transportation:

– Physiological and psychological behaviour of road

users

– Vehicles

Highway capacity and level of service

• Capacity and Level of service are two related terms.

• HOW??

Highway capacity and level of service

• The capacity of highways may be described as its ability to accommodate

traffic.

• Highway capacity itself is limited by the physical features of the highway,

which do not change unless the geometric design of the highway changes.

Highway capacity and level of service

Road type Standard

Maximum

Dual: 2-lane 2400 3200

Dual: 3-lane 3600 4800

Single: 10m

wide

1900 2300

Single: 7.3m

wide

1200 1600

Design flows for all-purpose dual/single carriageway roads

(Peak hourly flow( veh/hour/carriageway ))

Highway capacity and level of service

Factors affecting capacity:

Lane width:

Alignment:

Gradient:

Traffic composition:

Allowed for by converting to passenger car equivalents.

Highway capacity and level of service

• A term closely related to capacity and often confused

with it is service volume.

• When capacity gives a quantitative measure of traffic,

level of service or LOS tries to give a qualitative

measure.

Highway capacity and level of service

Highway capacity and level of service

Highway capacity and level of service

Highway capacity and level of service

Factors affecting transportation

• The success of transportation engineering depends upon

the co-ordination between the three primary elements,

namely the vehicles, the roadways, and the road users.

• Their characteristics affect the performance of the

transportation system and the transportation engineer should have fairly good understanding about them.

Factors affecting transportation

Components of Traffic System • Primary components of the traffic system:

• The general environment has an impact on traffic operations but it is difficult to assess.

• Traffic engineering would be great if these components had uniform characteristics

Road Users

• Very important as all transportation facilities have to be eventually used efficiently and safely by “Humans”.

• The study of human behaviour and the limitations of human performance in the varied circumstances of the road environment.

• Driver (user) response is a major component of planning and design of transportation systems.

• Human beings have a wide range of characteristics that influence the driving task.

Road Users

• Two of the important constituents of transportation

system are drivers and users/passengers.

• Understanding of certain human characteristics like

perception - reaction time and visual acuity and their

variability are to be considered by Traffic Engineer.

• Because of the variability in characteristics, the 85th

percentile values of the human characteristics are used

as standards for design of traffic facilities.

Road Users

• A major task is to find ways to provide drivers with information in a clear, effective, manner that induces safe and proper responses.

• Two important driver characteristics are

Q: Define the driving task and list the factors affecting a road user.

Road Users

• Perception:

• Intellect:

• Emotion:

• Volition:

Road Users

• The factors affecting a road user in the driving task can

be divided into 3 main categories:

– Physiological factors

– Psychological factors

– Modifying factors

• Physiological factors include:

– Visual Intake of information of drivers and pedestrians

– visual acuity: data stored to be used afterwards and in

near future

– Stereoscopy

Road Users – Adaptation of the eye to varying illumination and

impaired night vision

– Colour blindness

– Posture and operating conditions leading to fatigue and

impaired performance

– hearing

• Psychological factors include:

– Aggression

– Risk taking

– Exhibitionism

– Lack of concentration

• Modifying factors include:

– Age, injury and disease

– Fatigue

Road Users

– Alcohol and drugs

– Illness

– Weather

– Posture

– Motivation

– Environmental influences

– Education

– Conditioning

Road Users

Visual acuity

• relates to the field of clearest vision. The most acute vision is within

a cone of 3 to 5 degrees, fairly clear vision within 10 to 12 degrees

and the peripheral vision will be within 120 to 180 degrees.

• This is important when traffic signs and signals are placed, but other

factors like dynamic visual acuity, depth perception etc. should also

be considered for accurate design.

Road Users

Perception-reaction time

Four distinct processes:

- Detection: recognition that something requiring response is present

- Identification: driver acquires sufficient information concerning the object or condition to allow the consideration of a proper response

- Decision: analyse the information and make a decision about how to respond

Road Users

Perception-reaction time

- Response: physical response that result from the decision

• Perception-reaction time depends on many factors: age, gender, medical condition, alcohol and drug use, fatigue, sleep deprivation, emotional condition, type of vehicle, experience and knowledge, etc.

Road Users (contd.)

Perception-reaction time

- The American Association of State Highway and Transportation Officials (AASHTO) recommends 2.5 sec for design value.

- For signal timing, the Institute of Transportation Engineers (ITE) recommends 1.0 sec.

- For specific situations, a longer time may be recommended.

- ORN 6- 2.0 sec

Road Users

Perception-reaction time

- Reaction distance: distance the vehicle travels while the driver goes through the process of perception reaction time

Road Users Pedestrians

• A critical safety problem in the street and highway system involves the interactions of vehicles and pedestrians.

• For pedestrians, it has to be considered:

- Walking speed for signalized intersections

- Gap acceptance for unsignalized intersections-

Vehicles • For purpose of geometric design, motor vehicles into

main categories (ORN 40- Table 1):

- Passenger cars

-Motorcycles

- Small/ Large Buses

- Light/ Medium/ heavy goods

- Others: Tractors, road rollers

• Critical vehicles properties that need to be considered:

Vehicles

• The vehicle has less variable characteristics than the

road users, and these factors can be legislatively

controlled within defined limits.

• Legislation can impose limits on the overall performance,

weight, size, as well as specifying the minimum

requirements for individual items of equipment such as

brakes, lighting and indicators.

• Operation and performance of vehicle

Vehicles Turning Characteristics of vehicles

- Low speed turns (≤ 10 mph) (50 mph is equivalent to 80 km/h)

• Limited by the characteristics of the vehicle (minimum radius allowed). AASHTO specifies minimum design radii for each of the design vehicles

- High speed turns (≥ 10 mph)

• Limited by the dynamics of side friction between the roadway and the tyres, and by the superelevation of the roadway.

Acceleration performance of vehicles

• Passenger cars – high acceleration rates

• Heavy vehicles – lower rates of acceleration

Vehicles

Braking performance

• Most critical performance characteristics: braking performance.

• The time and distance required to stop

• Primary consideration in virtually every aspect of traffic system design and operation.

• Factors: vehicles braking system, type & condition of tyres