5 modal split-traffic assignment-( Transportation and Traffic Engineering Dr. Sheriff El-Badawy )

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المنصورة-قسم الهندسة المدنية –معهد مصر العالي للهندسة والتكنولوجيا

Dr. Sherif El-Badawy

3rd Year Civil

Transportation and Traffic Engineering

Modal Split and Traffic

Assignment


Objectives & Goals

Future planning & land

use parameters

Future transport

facilities

Transportation Generation

Transportation distribution

Model split

Traffic assignment

Is the level of

service satisfactory

Are the objectives

achived

YES

YES

No

Ch

an

ge F

utu

re tr

an

spo

rt f

aci

litie

s

No

Ch

ang

e L

and

use

& p

olic

ies

Present planning &

socio – economic data

Present transport

facilities

Present travel pattern

End

Survey phase

Comprehensive Urban Transport Planning Process

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المنصورة-قسم الهندسة المدنية –معهد مصر العالي للهندسة والتكنولوجيا 3

Modal SplitDefinition:

• Is the process of separating person-trips by the

mode of travel.

• Usually expressed as a fraction, ratio, or

percentage of the total number of trips.


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Characteristics of the trip

Trip purpose, trip length.

Household Characteristics

Income, Car ownership, family size and composition.

Zonal Characteristics

Residential density, concentration of workers,

distance from CBD.

Network Characteristics

Accessibility ratio, travel time ratio, travel cost ratio.

Factors Affecting Modal Split

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Mode choice depends on:

• Travel time (in Vehicle + waiting)

• Travel cost

• Socio-economic status of user (income,

family size,….)

• Level of travel service (comfort, Safety,

speed, ……..)

The variable defining the model is a function

called the Utility Function


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Utility Function (Multi-Nomial Logit function)

P%(m) = the probability of choosing mode (m) of transportation

e = exponential base

um = the utility function of different modes

if we assume an urban area with bus, car and taxi modes

% for bus mode will be:

% for car mode will be:

% for Taxi mode will be:

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Utility Function

The general utility function is usually of the form:

Where:

Km = mode specific constant

tm = in vehicle time (minutes-one way)

xm = out of vehicle time (minutes-one way)

d = distance (miles - one way)

cm = cost (piasters - one way)

Y= annual income (pounds)

m is A for Auto, B for Bus, T for Taxi, t for train


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Considering a user choosing between two modes an auto

and a public Bus considering the following situation:

If d was 7 miles. And it

is also given that the

annual income of the user

is 3000 pounds.

Bus Auto

tB= 14 mintA= 11 min

xB = 8 minxA = 5 min

cB = 50 piasterscA = 122 piasters

Example

And his value parameters as collected from data collection are:

K1 = -0.03 k2 = -0.34 k3 = -50

The values of the mode specific constants are assumed to be

kA = -0.13 kB = zero

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Solution

uA = -0.13 – 0.03 (11) - 0.34 (5/7) -50 (122/3000)

= -0.13 – 0.33 - 0.243 – 2.03 = -2.74

uB = 0 – 0.03 (14) - 0.34 (8/7) -50 (50/3000)

= 0 – 0.42 – 0.39 – 0.83 = -1.64

P%A = 25% and in the same way we get P%B = 75%

Bus Auto

tB= 14 mintA= 11 min

xB = 8 minxA = 5 min

cB = 50cA = 122

kB =-0kA =-0.13

k1 = -0.03 k2 = -0.34

k3 = -50


Review

(Modal Split)

(Traffic Assignment )

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Which route

will be chosen

by the user?


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Traffic Assignment

Definition: is the stage in the transportation

planning process where in the trip interchanges

are allocated to different parts of the network

forming the transportation system.

In this stage:

i. The route to be travelled is determined.

ii. The inter-zonal flows are assigned to the

selected routs.

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General principals

• All assignment techniques are based on route

selection.

• The choice of the route is made on the basis of

a number of criteria as Journey time, length,

cost, comfort, convenience, safety.

(Travel Resistance)

• Journey time is often considered the sole

criterion since length and cost can be

considered as a function of time in most cases.


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Transport LinkAs a first step, the highway network is described by a

system of links and nodes.

A link is a section of a highway network between two

intersections.

A node is either a centroid of a zone or the intersection of

two or more links.

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Link-Node Map for Highway System


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Traffic Assignment Methods

All or Nothing

Capacity Restraint

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All or Nothing Method

Most commonly used method.

Example: Minimum Path Tree

All traffic will choose the route where the travel

resistance is least.

Travel Resistance:

• Time

• Distance

• Cost

• Generalized Cost


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222523 24 26

15

11

1819

20 21

16

12

17

1

1314

(4)

(4)

(2)

(2)

(2) (2)

(2) (2) (2)

(2)

(2)

(1)

(1)(1)

(5)

(3)

(3)

(3)

(3)(3)

(5)

(5)

Example

(3)

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Starting from centroid 1 we go to each connecting link and

choose the least travel time

T1-20 = 3 T1-17 = 3

the time is the same , if we begin with the node with lower

number node 17 is noted:

T1-17-19 = 5 T1-17-16 = 5 T1-17-16 = 6

The next closest node to centroid 1 is 20

T1-20-19 = 4 T1-20-25 = 6 T1-20-21 = 7

There are two routes to reach 19 from centroid 1, i.e. 1-17-19

and 1-20-19. the rout 1-20-19 is shorter in time, therefore is

chosen


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222523 24 26

15

11

1819

20 21

16

12

17

1

1314

(4)

(4)

(2)

(2)

(2) (2)

(2) (2) (2)

(2)

(2)

(1)

(1)(1)

(5)

(3)

(3)

(3)

(3)(3)

(5)

(5)

The process is repeated until all nodes have been covered by the shortest path. The minimum path tree for this highway network is given in figure

(3)

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All or Nothing Assignment:

• All trips are assigned on the shortest route which is the

minimum travel time, cost, distance, …..between zones.

• Simple and inexpensive to perform.

• Does not take into account the effect of traffic congestion:

• Assumes there is no travel time change due to increased traffic.

• Flow patterns could be unrealistic.

• Can be used for special cases (under saturated traffic, ..etc)


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132 1

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Find the shortest path from node 1 to all other nodes (from Garber and Hoel)

Yellow numbers represent link travel times in minutes3

Here’

s

how

…

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FINAL

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Capacity Restraint Method

• Capacity restraint assignment is a process inwhich the travel resistance of a link isincreased according to a relation between thepractical capacity of the link and the volumesassigned to the link.

• Because of the iterative nature of thecalculations involved, the capacity restrainedmethod is carried out entirely by computer.

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Capacity Restraint Techniques

Smock Method Bureau of Public Roads Method

Where:

To original travel time

TA adjusted travel time

e exponential base

V assigned volume

C link capacity


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DHV = k * ADT

Where

DHV = Design Hourly Volume

k = Constant called Peak hour factor

ADT = Average Daily traffic

We get the required number of lanes according to the

Design Hourly Volume from the equation

No of lanes = DHV ÷ Lane capacity