3b traffic studies

7/30/2019 3B Traffic Studies

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KAAF UNIVERSITY COLLEGE

Civil Engineering DepartmentCollege of Engineering

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Transportation Engineering ICIV 367

Lecture 3B_ Traffic Studies

Kwasi Agyeman Boakye ( [email protected])

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Accident Studies

Is it really accident or road crash?

Accident: An occurrence involving one or more transport vehicle in a collision that results inproperty damage, injury or death.

Crash implies the collision could have been prevented or its effect minimized by modifyingdriver behaviour, vehicle design, roadway geometry or the travelling environment. Nonethelessaccident is more often used.

Accidents can be categorized into Casualty Accidents or Damage Only Accidents. Undercasualty accidents we may have;

Fatal Accident: It is one that involves a person who dies as a result of an injury sustained inthe accident (usually within 30days).

Serious Accident: It involves a person who is detained in hospital as an in-patient, or whosuffers any of these; fracture, concussion, internal injuries, severe cuts etc that require medicalattention.

Slight Accident: It involves a person who is only slightly injured, eg. Sprain, bruise, cut etcwhich is not considered severe and may require only roadside attention.

Damage Only Accidents are those that do not involve people sustaining injuries or casualties.2


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Factors Involved in Transport

Crashes

The Driver

Driver performance is a major contributor to many crash situations. Driver error may occurthrough inattention to the roadway and surrounding traffic, failure to yield the right of way and ortraffic laws. And, these error may come as a result of unfamiliarity with roadway conditions,traveling at high speeds, drowsiness, drinking, using a mobile phone etc.

The Vehicle

Mechanical conditions of vehicles may contribute to transport accidents. Faulty brakes in heavytrucks have caused crushes. Other reasons are failure of the electrical system, worn tires andlocation of the centre of gravity.

The Roadway

The condition and quality of the roadway, which includes the pavement, shoulders, curves,super elevations, intersections, and the traffic control system , can be a factor in a crash.

The Environment

The physical and climatic environment can be a factor. Weather is the most troubling sincetransport systems function best when the weather is sunny and mild and the weather is clear.Geography can also be an environmental factor. Mountain ranges, flood river plains, swollenrivers, and mud slides on the pavement have caused crashes.

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Accident Investigation and

Reduction

Accident Reduction can be achieved through the following;

Engineering: Using low-cost engineering proposals for improvement.

Education: Using publicity and the education/training of various groups of road users.

Enforcement: Proper and continuous enforcement of traffic laws.

These measures are implemented through Accident Reduction Programmes which are groupedas;

Single Site Approach

Mass Action Approach

Route Action Approach

Area Action Approach

To attempt using any of the approaches, four major investigative steps ought to be followed,namely;

1. Data collection, storage and retrieval

2. Identification of hazardous locations for further study

3. Diagnosis of the accident problem4. Final selection of sites to be included in remedial implementation programme

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Programme Approach to Accident

Reduction

Single Site Approach

Involves the identification of blackspot locations on the basis of the number of accidentsclustered at single locations within a given period of time (usually 3 years). Blackspot locationsare typically individual intersections, short lengths (300 500m) of roadway or small areas (100 200m square). For remedial measures to be taken at a blackspot the reactional level shouldbe already established and achieved. Reactional Level is the total number of accidents

required for subsequent investigation.

A large accident reduction (33%) and a high 1st year economic rate of return(ERR)(50%) shouldbe expected from remedying blackspots.

Mass Action Approach

Involves searching for sites that are clearly associated with a particular predominant type ofaccident for which there is a well-proven engineering remedy. Upon identification the sites canbe treated in a mass, by for instance resurfacing or surface dressing of skid sites. They areparticularly applicable to Education and Enforcement measures.

It is expected to achieve an average accident reduction of 15% at treated locations and 1st yearERR greater than 40%.

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Programme Approach to Accident

Reduction

Route Action Approach

In this approach blacksites ; sites with more accidents along a particular road than the norm forthat type of road and level of traffic usage, are identified from an analysis of traffic data collectedover a recent period (usually 1 3 years). Usually the search process involves dividing the roadinto section lengths of 0.5 1.5km.

Area Action Approach

Where accidents are sparsely scattered and do not fall within the category of treatment providedby the others previously mentioned, this approach is used, especially in urban areas. Studies isdone on the accident distribution in the area by comparing data between years 1, 2 or 3 anddiscrete areas identified. This could be accidents per unit area or population.

Area action programmes may result in interventions that reduce traffic movement on localaccess roads within the identified areas. They are expected to achieve an accident reduction of10% and a 1st year ERR of 10 25 % within each area addressed.

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Accident Investigation Step 1

Data Collection, Storage and Retrieval

Involves manual filing of each completed accident report form in the offices of the appropriatepolicy agency. Filed usually by date, name or number of route, location. Included in the file is abasic accident description, road features, environmental features, vehicle features, driverfeatures etc.

The data is often computerised and manipulated into accident ranking lists, tables, graphs andautomatic plots. Collision Diagrams present pictorial information on individual crashes at alocation. The type of collisions often recorded include Rear End, Head On, Side Swipe, LeftTurn, Right Angle and Out of Control. They have an advantage of giving information on thelocation of the crash, which statistical summaries do not.

In Ghana, accidents reported to the police are recorded on an Accident Report Form. TheBuilding and Road Research Institute (BRRI) collect these forms from the police and enter it intoa computerised accident database. They use (MAAPfive), a software, to analyse the data andproduce summary statistics.

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Collision Diagram

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Accident Statistics in Ghana

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Identification of hazardous locations for further study

This stage is concerned with sifting of the accident data to obtain a preliminary ranked list ofsites that are most likely to be susceptible to engineering treatment. This may be done throughseveral means;

Simple Numeric Criteria : This is based on the location with the highest accident figures. The

reactional level might be 12 accidents per defined spot that occurred within the last 3years, andany sites with more than that number require further investigations. Further ranking may bedone from most dangerous to least dangerous.

Exposure Rates: Due to the likelihood of the Simple Numeric Criteria favouring routes mosttrafficked , exposure rates become a very useful measure. This could be done by measuringeither the rate per million of entering vehicles (RMEVs) or rate per 100 million vehiclemiles (RMVM).

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RMEV= A x 1,000,000V

RMEV = crash rate per million enteringvehiclesA = number of crashes, total or by typeoccurring in a single year at the locationand V= ADT x 365This is often used at intersections

RMVM= A x 100,000,000

VMTRMVM = crash per 100 million vehicle miles oftravel.

A = number of crashes, total or by type at thestudy location, during a given period VMT=vehicle miles of travel during the period

Often used on stretch of highways with similargeometric and traffic characteristics


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Severity: This method involves listing each crash occurring at a site under one of the

following classes: fatal (F), personal injury (PI) and property damage (PD). Several weighing

scales are used but a typical one is Fatality = 12, Personal Injury = 3 Proper damage = 1.

For example, if 1 fatal crash, 3 personal injury crashes, and 5 property damage crashes

occurred during a year at a particular site, the severity number of the site is obtained as follows;

Severity Number = (12 X 1) + (3 X 3) + ( 1 X 5) = 26

The disadvantage in using this method is the large difference between the severity scales for

fatal and property damage crashes. This may be overemphasized. For example, a site with onefatal crash will be considered much more dangerous than a site with 9 property damage

crashes.

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Diagnosis of the Accident Problem

This stage involves the detailed analysis of the accident situation at each site. This is todiagnose the main causes of each accident and develop a remedial proposal that will obviatethe common dominant cause.

As part of the engineering analysis, collision and condition diagrams are often prepared

for each site to assist in the determination of the dominant accident causes, particularly ifphysical features are believed to be influencing the accident situation. A condition diagram is ascaled drawing or photograph illustrating the physical and environmental conditions at andabout the accident site e.g the geometric features affecting the site, and the locations of all signsand markings, pedestrian crossings, traffic signals, bus stops, parking spaces, sightobstructions, driveways, and fronting landuse.

Diagrams are supplemented with field visits which may simply involve taking skid-resistance

measurements, or it may require the use of a detailed check-list to determine the adequacy ofvarious physical design features.

In some situations it may require a comprehensive on-the-spot conflict study. TrafficConflict is an observable situation in which two or more road users approach each other inspace and time to such an extent that a collision is imminent if their movements remainunchanged.

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Final Selection

Solutions provided should go to either;

- Remove conflict causing the problem

- Improve the situation

- Reduce speed, thus reducing the accident risk and accident severity

It is the usual norm to proffer two or more remedial solutions of different cost for a given site.As such a chosen solution must be economically justified.Various economic methods are usedto resolve these and other issues of choice. One of them used is the first year of economic rateof return (FYRR).

FYRR = Accident Savings x 100

Capital Cost

For major remedial schemes with a significant maintenance and renewal cost elements(such as the installation of traffic signals) it will be better to use an economic assessmentmethod such as Net Present Value.

Even after economic the economic analysis it is necessary that approval and support besought from the implementation agency, the police and the public.

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Example

Scheme: Installation of 5 chevrons signs at a bend in order to reduce loss of control accidents.

Accident Record: Average of 5 casualty accidents per year ( 4 single vehicle loss of control and1 pedestrian/vehicle)

Estimated Accident Savings in First Year: 15% reduction in treatable accidents (0.6 accidents

prevented)Cost value per accident saved = $10,000

Capital cost of per sign = $ 20

FYRR(%) = 0.6 x 10,000 = 600%

20x5

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Examples of Accident Causes and

Countermeasures

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Pattern Probable Cause General Countermeasure

Create one-way street

Widen road

Provide left-turn signal phase

Prohibit left turns

Remove obstacles

Provide adequate channelizationProvide special phase for left turn

Provide left turn slots

Increase amber phase

Provide all red phase

Driver not aware of intersection Install/improve warning signs

Overlay pavement

Provide adequate drainage

groove pavement

Create left or right turn lanes

Prohibit turns

Increase curb radii

Inadequate roadway l ighting Improve roadway l ighting

Restricted sight distance

Inadequate protection for pedestrian

School crossing areaUnadequate phasing signal

Pedestrian-vehicle collisions

Examples of Accident Pattern, Probable Cause and Countermeasures

Left-turn head-on collision Large volume of left-turn

Restricted sight distance

Too short Amber phase

Rear-end collisions at unsignalized intersections

Slippery surface

Large number of turning vehicles


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Parking

It is roughly estimated that out of 8,760 hours in a year, the car runs on an average for 400hours, leaving 8,360 hours when it is parked. Every car owner would wish to park the car asclosely as possible to his destination so as to minimise walking. This results in a great demandfor parking space in the CBD and other areas where activities are concentrated. Parking has ineffect led to;

Congestion: One of the serious ill-effects of parking is the loss of street space and the

attendant traffic congestion. The capacity of the street is reduced, the journey speed dropsdown and the journey time and delay increase.

Accidents: The manoeuvres associated with parking and unparking are known to cause roadaccidents. Careless opening of the doors of parked vehicles, moving out of a parked positionand bringing a car to the parking location from the main stream of traffic are some of the

common causes of parking accidents.

Obstruction to fire-fighting operations: Parked cars obstruct the movement of fire-fightingvehicles and greatly impede their operations. They block access to fire hydrants and access tobuildings.

Environment: Stopping and starting of vehicles result in noise and fumes. Cars parked intoevery little available space debase the visual aesthetics.

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Parking Facilities and Terms

Parking facilities can be divided into two main groups; on street and off-street.

On-Street Parking Facilities: These are also known as Kerb facilities. Parking bays areprovided alongside the kerb on one or both sides of the street. These bays can be categorisedinto either restricted or unrestricted parking. Where it is unrestricted the duration of parking isunlimited and parking is free. And where it is restricted parking is limited to specific times of theday for a maximum duration. Parking at restricted facilities may or may not be free.

Off-Street Parking Facilities: These facilities may be privately or publicly owned. They includesurface lots and garages. Self- parking garages require that drivers park their own cars;attendant-parking garages maintain personnel to park the vehicles.

Parking Terms

Space

hour: is a unit of parking that defines the use of a single parking space for a period of1hr.

Parking Volume is the number of vehicles that park in a study area during a specific length oftime, usually a day.

Parking duration is the length of time a vehicle is parked at a parking bay.

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Parking Terms

Parking accumulation is the number of parked

vehicles in a study area at any specified time

The parking load is the area under the

accumulation curve between two specific

times. It is usually given as the number of

space-hours used during the specified timeperiod.

Parking Index is also called occupancy or efficiency. It is the ratio of number of parking spaceoccupied in a time duration to the total space available. It gives an aggregate measure of how

effectively the parking space is utilized.

Parking Index = Parking load X 100

Parking Capacity

Parking turnover is the rate of use of a parking space. It is obtained by dividing the parkingvolume for a specified period by the number of parking spaces.

T = number of different vehicles parked

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Methodology of Parking Study

The main steps are as follows;

(1) inventory of existing parking facilities

(2) collection of data on parking accumulation, parking turnover and parking duration,

(3) identification of parking generators, and

(4) collection of information on parking demand.

1. Inventory

Type and number of parking spaces at each parking facility

Times of operation and limit on duration of parking, if any

Type of ownership (private or public)

Parking fees, if any, and method of collection

Restriction on use (open or closed to the public)

Other restrictions, if any (such as loading and unloading zones, bus stops, or taxi ranks

Probable degree of permanency (can the facility be regarded as permanent or is it justtemporary facility?)

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Methodology of Parking Study

2. Collection of Parking Data

Accumulation. Accumulation data are obtained by checking the amount of parking duringregular intervals on different days of the week. The checks are usually carried out on an hourlyor 2-hour basis between 6:00am and midnight. The information obtained is used to determinehourly variations of parking and peak periods of parking demand.

Turnover and Duration. Information on turnover and duration is usually obtained by collectingdata on a sample of parking spaces in a given block. This is done by recording the license plateof the vehicle parked on each parking space in the sample at the ends of fixed intervals duringthe study period. The length of the fixed intervals depends on the maximum permissibleduration.

3. Identification of Parking Generators

The phase involves identifying parking generators (for example, shopping centers or transitterminals) and locating these on a map of the study area.

4. Parking Demand

Information on parking demand is obtained by interviewing drivers at the various parkingfacilities listed during the inventory. Information sought should include (1) trip origin, (2) purposeof trip, and (3) drivers destination after parking. 20


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Analysis of Parking Data

Analysis of parking data includes summarizing, coding and interpreting the data so that therelevant information required for decision making can be obtained. The relevant informationincludes the following:

Number and duration for vehicles legally parked

Number and duration for vehicles illegally parked

Space-hours of demand for parking

Supply of parking facilities

The space-hours of demand for parking are obtained from the expression

where D = space vehicle-hours demand for a specific period of time

N = number of classes of parking duration ranges

ti = midparking duration of the ith vehicle

ni = number of vehicles parked for the ith duration range

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Analysis of Parking Data

The space-hours of supply are obtained from the expression

S = practical number of space-hours of supply for a specific period of time

N = number of parking spaces availableti = total length of time in hours when the ith space can be legally parked on duringthe specific period

f = efficiency factor

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Question

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The owners of Rawlings Park, Accra have observedthat 20% of those wishing to park are turned backevery day during the open hours of 8 am to 6pmbecause of lack of parking spaces. An analysis ofdata collected at the garage indicates that 60% ofthose who park are commuters, with an averageparking duration of 9hrs and the remaining areshoppers, whose average parking duration is 2hrs.If 20% of those who cannot park are commutersand the rest are shoppers, and a total of 200vehicles currently park daily in the garage,determine the number of additional spaces requiredto meet the excess demand. Assume parkingefficiency is 0.90.

Ans.Calculate the space-hours of demand using

The demand for those being served include;Commuters = 0.6x 200x9 = 1080 space-hrShoppers = 0.4x200x2 = 160 space-hr

If those unserved parking are 20%, then thoseserved are 80%. And if 80% represent 200 vehicles

parking then 20% will represent;=20x200 = 50 vehicles turned back.80

The demand of the 50 vehicles turned back;Commuters = 0.2x50x9 = 90 space-hrShoppers = 0.8x50x2 = 80 space-hr

Total Demand ( Served and Unserved)

= 1080+160+90+80 = 1410 space-hrs

Total Served = 1080+160= 1240 space hrs

Number of space hours required= ServedUnserved=1410 1240 = 170 space -hrs


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Question

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Number of parking spaces required for a parkingdemand of 170 space-hrs;

Where f = 0.9 and ti = 10 hours since vehicles aregoing to be parked from 8am to 6pm, availablespace will be;

0.9x10xN=170N = 18.89

This can be rounded as 19 additional spaces will be

required.

Assignment.From the previous question, the owners of theRawlings Park decided to introduce the 19additional spaces which now allows the unservedvehicles now to be served. Determine;i. The parking turnover.ii. The parking index.


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Parking Dimensions

Usually a standard dimensions of a car is taken as 5 x 2.5 and that for a truck is 3.75 x 7.5 forparking.

For residential plot area from 101 to 300 sq.m only community parking space is required. Forresidential plot area from 500 to 1000 sq.m, minimum one-fourth of the open area should bereserved for parking. For offices, at least one space for every 70 sq.m should be kept aside for

parking facility of vehicles. One parking space is enough for 10 seats in a restaurant where astheatres and cinema halls need keep only 1 parking space for 20 seats. Thus the parkingrequirements are different for different land use zones.

In Ghana, Town and Country Planning has planning standards for vehicle parking which is oftenused. For different classes of residential facilities there are a number of vehicle parking space

required. Also for each given land use there has been provided one parking space for a definedunit area providing a particular service.

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On Street Parking

Parallel parking: The vehicles are parked along the length of the road. Here there is nobackward movement involved while unparking the vehicle. Hence it is the most safest parkingfrom the accident perspective. But it consumes the maximum kerb length and therefore only aminimum number of vehicles can be parked for a given kerb length. Since it consumes leastwidth of the road it produces the least obstruction to the on-going traffic on the road.

30 Degrees parking: Here more vehicles can be parked compared to parallel parking. Also

there is better maneuverability. It causes minimum delay to traffic.45 and 60 Degrees parking: As the angle of parking increases, more number of vehicles canbe parked. Hence compared to parallel and thirty degree parking, more number of vehicles canbe accommodated in this type of parking.

Right Angle Parking: Here the vehicles are parked perpendicular to the direction of the road.Hence it consumes maximum width. Kerb length required is very little. Hence there are chances

of severe accidents. Also it causes obstruction to the road trac. But it can accommodatemaximum number of vehicles.

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Off Street Parking

There will be some area exclusively allotted for parking which will be at some distance awayfrom the main stream of traffic. Such a parking is referred to as off-street parking. They may beoperated by either public agencies or private rms.

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