traffic conflict study- right hand drive1.pdf

7/24/2019 Traffic Conflict Study- Right Hand Drive1.pdf

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TRAFFIC CONFLICT

STUDIES


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Background


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Traffic engineers use accident data to determine:

Which locations are most hazardous.

What kinds of hazards are present.

Whether a change in the design or operation

has been effective.

Background


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Problems with accident data?

Not all accidents are reported.

Due to manpower & budget limitations, there is

a trend by police agencies not to report property

damage only accidents.

Incomplete accident data & errors makes

identification of roadway hazards less accurate.

Background


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Accidents are fairly infrequent & happen

sporadically.

A long time is needed to collect enough accident

data to be useful.

Background


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Statistical problem that many people do not

appreciate ?

Nos. of accidents at one intersection from yr to yr :

10 24 12, 20, 18, 14, 20 10, 14

If one looked at only one or two years, one would not

know what is typical.

Is it 10 to 24 per year, or is it 20 to 10 per year?

Background


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Thus,

what other traffic indicators

are useful ???

Background


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TCT is perhaps the most developed indirect

measure of traffic safety.

TCT is the ability to register the occurrence ofnear-accidents directly in real-time traffic.

Offers a faster & more representative way of

estimating expected accident frequency &

accident outcomes.

Traffic Conflict Technique (TCT)


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The main criticisms?

Concern issues of reliability & validity.

Whether accident risk is actually being measured

in a satisfactory manner.

Traffic Conflict Technique (TCT)


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Conflict technique emanates from research

originally at the Detroit General Motors laboratory

in the late 1960s for identifying safety problems

related to vehicle construction (Perkins & Harris,1968).

History of TCT


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Use of this technique soon spread to different

parts of the world.

TRL in England soon recognised the need to add

a subjective scale for observed conflicts as ameasure of severity (Spicer, 1973).

This technique was based on observer

judgements using time-lapse filming, therebyproving costly & time-consuming.

History of TCT


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Swedish Traffic Conflict Technique (STCT) was

developed at Lund University in different projects

during the 1970s & 1980s before finally reachingpresent day level of development in 1987 (Hydn,

1987).

History of TCT


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Swedish technique focuses on situations where

two road-users would have collided had neither of

them made any kind of aversive manoeuvre.

History of TCT


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The point at which the aversive action is taken

is recorded through observation as the

Time-to-Accident(TA).

TA value together with the conflicting speed is

used to determine whether or not a conflict is

serious.

History of TCT


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Collision diagram


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Conflict diagram


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TCT

Reference:

Traffic Conflict Techniques for Safety and Operations

(Observers Manual)

US Department of Transportation,

Federal Highway AdministrationJanuary 1989

Definitions


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Primary requirement of a traffic conflict is that

the action of the 1stuser places the other user on

a collision path, unless evasive action is taken by

the other user to avoid the accident.

Generally, the road users are motorists,

but the definition also includes

pedestrians, bicyclists & motorcyclists.

General Definition


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General definition

Rule Out

actions that nearly all drivers take

under the same conditionssuch as:

normal stopping for a STOP sign

OR

red traffic signal.

General Definition


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Collisions & Near miss situations that occur

without evasive manoeuvres,

or when the evasive action is inadequate

are also recorded as conflictsunder the general definition.

General Definition


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An intersection traffic conflict is described as an

event involving the following stages.

Stage 1:The 1stvehicle makes a maneuver;

e.g., pulling out from the cross street.

Stage 2:

A 2nd vehicle is placed in danger of a collision.

General Definition


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Stage 3:

The driver of 2nd vehicle reacts by braking

or swerving.

Stage 4:

The 2nd vehicle then continues to proceed

through the intersection area.

General Definition


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Note:

The last stage is necessary to convince the observer

that the 2nd vehicle was actually responding to

the manoeuvre of the 1st

vehicle,

and not,

for example to a traffic control device or nearby

driveway or median opening.

General Definition


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Evasive manoeuvre taken by 2nd vehicle

is evidenced by obvious braking or swerving.

Braking is usually observed as brakelight

indications.

Noticeable diving of the vehicle or squealing of

tyres in the absence of brake lights is acceptable

evidence of an evasive manoeuvre.

General Definition


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Basic set of conflict definitions

were developed for intersections,

corresponding to thedifferent types of manoeuvres &

related accident patterns.

Operational Definition


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Similar to the manner in which

accidents are grouped

by type of collision,

traffic conflicts are categorised

by type of manoeuvre.



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Primary types of intersection conflicts:

1. Same Direction,

2. Opposing Right-Turn,3. Cross Traffic,

4. Left-Turn-On-Red,

5. Pedestrian.

6. Secondary.



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The primary types of intersection conflicts are:

1. Same Direction



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Same Direction Conflicts:

A same-direction conflict occurs when 1stvehicle

slows and/or changes direction and places

the following vehicle in danger of a rear

end

collision.

2nd vehicle brakes or swerves to avoid the

collision, then continues to proceed through theintersection area.



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4 basic types of

SameDirection Conflicts ??



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i) Right-Turn, Same-Direction Conflict

A right-turn, samedirection conflict occurs when

the 1stvehicle slows to make a right turn, thus

placing a 2ndfollowing vehicle in danger of

a rear-end collision (see Figure 1).



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X

observer

Fig. 1: RT, same direction conflict


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ii) Left-Turn, Same-Direction Conflict

A left-turn, samedirection conflict occurs when

the 1stvehicle slows to make a left turn, thus

placing a 2ndfollowing vehicle in danger of a

rear-end collision (see Figure 2).



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X

observer

Fig. 2: LT, same direction conflict


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iii) SlowVehicle, Same-Direction Conflict

A slow-vehicle, same-direction conflict occurs

when the 1st

vehicle slows while approachingor passing through the intersection, placing a

2ndfollowing vehicle in danger of a rear-end

collision (see Figure 3).



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X

observer

Fig. 3: Slow vehicle, same direction conflict


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iv) LaneChange Conflict

A lane-change conflict occurs when the 1stvehicle

changes from one lane to another, thus placing a

2ndfollowing a vehicle in the new lane indanger of a rear-end or sideswipe collision

(see Figure 4).



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X

observer

Fig. 4: Lane change conflict


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2. Opposing Right-Turn



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i) Opposing Right-Turn Conflict

An opposing right-turn conflict occurs when an

oncoming vehicle makes a right turn, thus placing

a 2nd vehicle, going in the other direction,

in danger of a head-on or broadside collision

(see Figure 5).



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Xobserver

Fig. 5: Opposing RT conflict


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3. Cross Traffic



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Cross-Traffic Conflicts

A cross-traffic conflict occurs when a vehicle on

the cross street turns or crosses into the path of

a 2nd vehicle on the main street who has the

right-ofway and places the 2nd vehicle in

danger of a rear-end, sideswipe, or broadside

collision, then proceeds through the intersection

area.



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Cross-traffic conflicts can occur from vehicle

manoeuvres on the left-hand and/or right-hand

cross street approach.



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a) Cross-Traffic Conflicts

From Left Cross Street Approach


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i) Left-Turn, Cross-Traffic-From-Left Conflict

A leftturn, crosstrafficfrom-left conflict occurs

when a vehicle on the lefthand cross street makes

a left turn, thus placing a 2nd vehicle on themain street in danger of a broadside or rear-end

collision.

(see Figure 6).



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Xobserver

Fig. 6: LT, cross-traffic from left conflict


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ii) Right-Turn, Cross-Traffic-From-Left Conflict

A rightturn, crosstrafficfrom-left conflict occurs

when a vehicle on the lefthand cross street makes

a right turn, thus placing a 2nd vehicle on the main

street in danger of a broadside collision

(see Figure 7).



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Xobserver

Fig. 7: RT, cross-traffic from left conflict


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iii) Through, Cross-Traffic-From-Left Conflict

A through, cross-trafficfrom-left conflict occurs

when a vehicle on the left-hand cross street

crosses in front of a 2nd vehicle on the main

street, placing it in danger of a broadside collision

(see Figure 8).



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Xobserver

Fig. 8: Through, cross-traffic from left conflict


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b) Cross-Traffic Conflicts

From Right Cross Street Approach


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i) Left-Turn, Cross-Traffic-From-Right Conflict

A left-turn, cross-traffic-fromright conflict occurs

when a vehicle on the right-hand cross street makes a

left turn across the center of the main streetroadway and into an opposing lane, thus placing a

vehicle in that lane in danger of a head-on collision

(see Figure 9).



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Xobserver

Fig. 9: LT, cross-traffic from right conflict


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ii) Right-Turn, Cross-Traffic-From Right Conflict

A right-turn, cross-trafficfrom-right conflict occurs

when a vehicle on the right-hand cross street makes

a right turn, thus placing a 2nd vehicle on the

main street in danger of a broadside or rear-end

collision (see Figure 10).



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Xobserver

Fig. 10: RT, cross-traffic from right conflict


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iii) Through Cross-Traffic-From-Right Conflict

A through, cross-traffic-from-right conflict occurs

when a vehicle on the right-hand cross street crosses

in front of a 2nd vehicle on the main street

placing it in danger of a broadside collision

(see Figure 11).



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Xobserver

Fig. 11: Through cross-traffic from right conflict


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4. Left-Turn-On-Red



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Left -turn-onred (LTOR) conflicts occur when a

LTOR vehicle makes a turn and crosses into the lane

of a second vehicle which has the right-of-way.

The driver of the 2nd vehicle brakes or swerves

to avoid a broadside, sideswipe, or rear-end

collision, then proceeds through the intersection

area.


Left-Turn

On

Red Conflicts


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i) Opposing Left-Turn-On

Red Conflict

An opposing left-turn-on-red conflict can only

occur at a signalized intersection with a protected

rightturn phase.It happens when an oncoming vehicle makes a

leftturn-on-red during the protected right-turn

phase, thus placing a right turning, 2nd vehicle

(which has the right-of-way) in danger of abroadside or rear-end collision.

(see Figure 12).



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Xobserver

Fig. 12: Opposing LT on red conflict


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ii) Left-Turn-On-Red-From-Left Conflict

A left-turn-on-red-from-left conflict is a special

category of the left-turn, cross-traffic-from-left

conflict (see Figure 13).The leftturn onred conflict occurs only at

signalized intersections when a LTOR vehicle on

the left-hand cross street makes a LTOR manoeuvre

and places a 2nd

vehicle on the main street indanger of a sideswipe or rearend collision.



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Xobserver

Fig. 13: LT on red traffic from left conflict


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5. Pedestrian



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Pedestrian Conflicts

They occur when a pedestrian (the road user causing

the conflict) crosses in front of a vehicle that hasthe rightofway, thus creating a possible collision

situation.

The vehicle brakes or swerves, then continues through

the intersection area.



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Any such crossing on the near side or far side of the

intersection (see Figures 14 and 15) is liable to be a

conflict situation.



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Xobserver

Fig. 14: Pedestrian, far-side conflict


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Xobserver

Fig. 15: Pedestrian, near-side conflict


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However, the pedestrian movements on the

right and left sides of the intersection are

generally not considered to create conflict

situations if the movements have the rightof

-way, such as during a WALK phase.



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6. Secondary



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Secondary Conflicts

In all of the foregoing conflict situations, when the

2nd vehicle makes an evasive manoeuvre, it may

place another road user (3rd vehicle) in dangerof a collision.

This type of event is called a secondary conflict.



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Nearly always, the secondary conflict will look much

like a slow-vehicle, samedirection conflict or a

lane-change conflict.



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The difference is that,

in a secondary conflict the

3

rd

vehicle is responding to a 2

nd

vehicle that,

itself, is in a conflict situation.

See examples in Figures 16 and 17.



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Xobserver

Fig. 17: LT, cross traffic from left secondary conflict


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By definition,

only one secondary conflict for any

initial conflict should be counted.

Even if a whole line of cars stops because the 1st

vehicle turns left, the event would be recorded as

one left-turn same-direction conflict and one

secondary conflict.



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Conducting the

Survey


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Arrive at site

Select observation points

Check basic traffic patterns

Ready to conduct the survey.

Conducting the Survey


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Time scheduling:

~survey several hours or several days?

~ observation process is still in 1-hour blocks.

Traffic engineer in charge determine:

~how many hours?

~ which hours to be used for conflict counting.

(see Figure 18for site diagram)



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The Conflict Form



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A general-purpose form for recording traffic conflict

is shown in Figure 19.

The heading information, which is self-explanatory,should be filled out in advance.



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Diagram in upper right corner indicates the

approach leg numbering system.

Traffic approaching from North is on Leg 1, etc.

separate traffic conflict form

should be used for

each approach leg



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Recording period, e.g. 15-min. must be recorded at

the top of the form.

Start time should be recorded for each 15-min.

count (or other period specified by the engineer)in the first column.

If, for any reason, the count was for

other than the prescribed count period,record the actual time in the left margin.



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Typically, two observers are used to obtain

conflict data at intersections.

One observer could count conflicts for bothapproaches by periodically alternating his

recording periods.



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Collecting Other Data


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Traffic conflict counts are not meaningful unless they

can be related to the existing site conditions.

The site data needed may include:

i. physical inventory,

ii. intersection diagram,

iii. signal timings,

iv. photographs,

v. on-site observation report,vi. traffic volumes.


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i) Physical Inventory

Signs, signals, pavement markings, type of

intersection, and other useful information are

noted in physical inventory.

Also include:

posted speed limits on all approaches.


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iii) Signal Timing

This includes:

cycle length, green, amber, red times

for all approaches.

Any separate phases, such as a right-turn phase,

should also be given.


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iv) Photographs/Videos

At least 5 pictures per approach, taken from the

drivers viewpoint.

2 cross-corner photographs made from the

stop line as though a driver were looking

left & right at the approaching cross traffic.


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v) On

Site Observation Report

During the course of the day,

the observer should note any apparent operational

& safety deficiencies of the intersection.


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Time Schedule for

Collecting Other Data


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Can be completed before conflict observations

begin:

physical inventory,

intersection diagram,

signal timing,

photographs


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On-Site Observation Report should be

completed only after the conflict count is finished.

At this time, the observer should have a goodunderstanding of how the intersection operates &

some of the problems associated with it.


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Special Problems


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Other problems may also occur :

Disruptive events such as accidents,

stalled vehicles, or police arrests that

could alter normal traffic flow patterns


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Safety Considerations


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Completing the Survey


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After completing the field work, the observer shouldprovide the data to engineer for analysis and

interpretation.

Unusual problems or special conflict situationsshould be discussed personally with the engineer

whenever possible.


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Some common analysis??

Conflict diagram

% Conflict types & vehicles involved

Dominant conflict types VS deficiencies at

intersection

Average conflicts per hour?

Average conflicts per hourly entering volumes


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End

traffic conflict study- right hand drive1.pdf

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