N_Traffic Management Chap 13

8/13/2019 N_Traffic Management Chap 13

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Traffic Management

Hossein Naraghi

CE 590 Special Topics

Safety

January 2003

Time spent:13 hrs


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Road network and functionalhierarchy

Traffic management is a process ofadjusting or adapting the existing road

network to improve traffic operationswithout major construction

Traffic management objectives may include

Traffic efficiency

Road capacity Improved environment amenity

Reduced noise


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Traffic management objectives(continued)

Enhanced access

Better access for particular group of roadusers

Pedestrians

Bicyclists

Freight vehicles

Road safetyThese objectives may potentially be inconflict of each other, so priorities mayhave to be determined.


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Road functions and functionalhierarchy

The use of traffic management to pursuesafety objectives must take place in the

context of a clear view of the functions of aroad network, which is referred to asfunctional hierarchy.

Roads function is the prime determinant ofthe management of any given road withinthe overall network.


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Road functions and functionalhierarchy (continued)

There are essentially two needs from aroad function point of view The traffic movement function

The role of roads in providing a means to transferpeople and goods from one place to another; theseroads constitute the arterial road network

The access function The role of these roads is providing access to abutting

properties and land uses; these roads constitute thelocal road network

Ideally each road would perform one of these

functions, but in practice there are many roads

that perform both functions


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The only roads that do not have an accessfunction are those which are access

controlled The only access is via ramps at interchanges

Freeways

Access from abutting property is oriented away

from the road Sometimes done with distributor roads in residentialneighborhoods

Access via frontage roads paralleling an arterialroads


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The roads which carries both functions ofaccess and mobility, create a majorchallenge for traffic management Tend to have very poor crash record as a result

of their mixed and inherently conflictingfunctions

People living and working along them seek to

use them for Access purposes Exiting and entering properties Parking in the street

Have significant pedestrians and bicycle

activities in residential areas


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Road hierarchy as a networkplanning tool

The basis for any traffic management plan isusually the development of road hierarchy andagreement to it by various stakeholders.

Main objectives

Prevent residential areas being used by throughtraffics

Influence driver behavior to follow planned routes at

moderates speeds, with proper attention being givento pedestrians and bicyclists

Use physical devices in support of legal regulations inorder to overcome the lack of enforcement for such

measures as speed limits, one way streets andturning prohibitions.


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Road hierarchy as a networkplanning tool (continued)

Gunnarsson defines three classes of road

F (exclusive foot space)

C (local street) T (exclusive transport space)

This provides a conceptual framework for trafficcalming

TF CF/C

T/C

Gunnarson describes local street (zone C) and twotransition zones (F/C and T/C) as traffic calming space


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Lessons for new network(continued)

Research for the safety aspects of roadnetwork design summarized the key

network planning principles for newresidential areas as follows

Strict differentiation of streets according to theirtraffic function leads to safer residential areas

Distribution of traffic into residential area withmultiple access from a ring road is safer thancentral distribution


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Lessons for new network(continued)

Full segregation of vehicle, pedestrians andbicycle movements is accompanied by very low

accident rates Cul-de-sac streets are safer than loop streets,

which in turns are safer than ordinary throughstreets

Crash rate is minimized where frontage accessis prohibited


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Traffic management or trafficcalming

There have been many differentinterpretations of traffic calming in differentcountries

A useful resolution of this conflict ofinterpretation has been proposed byBrindle

Traffic calming is used encompasses two axes The scope of the measure

Local, intermediate or citywide

The type of measure Use of physical devices and regulations

Social/cultural change


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Area treatments

Area treatments divided into twodistinct types

Applicable to low density residentialdevelopment

Aim to control speeds which are in excessof a statutory speed limit of around 30-35

mph

Used in higher density cities

Aim is to reduce speed to around 20 mph


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Area treatments (continued)

Safety effectiveness Traffic management schemes are usually successful

in reducing

Vehicle speeds

Sometimes traffic volumes

But whether they increase objective safety; asmeasured by accident, is open to question

An in depth analysis of accident pattern within casestudy area in Sydney concluded that

Many of traffic management devices used extensivelyin local area fail to resolve the dominant types ofaccidents occurring on local streets


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The dominant types of accident were

Cross traffic

Right-through Rare end

Hit parked vehicles

Local area traffic management schemes may not

be as effective in reducing accidents as manypractitioners believe

However, there were several studies which haverevealed the positive effects of traffic

management schemes


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In either case, treatments typically involvesome or all of the following

A form of gateway treatment using signs andtown entry features to

Emphasize the changed status of the road

Encourage slower driving

Provide a sense of identity

Speed restricting devices

Road narrowing

Road humps

Enhanced pedestrians and bicyclists facilities


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Improved parking especially for bypass towns

The town can rejuvenate as a business center whenthrough traffic is removed

Change traffic furniture like lighting and seating Enhanced signing

A roundabout at the start and end of the town

To ensure traffic slows

Narrower lanes

Change in road surface including color and texture

Additional traffic control measures

Traffic signal

Pedestrian crossing


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Problems and difficulties

There is a negative response from affected

parties which needs to be considered Brindle (1992) reported a citizen group

opposition to traffic calming (especiallyspeed humps) in the following terms

They discriminating against law-abiding roadusers

They are dangerous to bicyclists and motorcyclists, especially when wet


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Problems and difficulties(continued)

They cause unnecessary wear and tear on motorvehicles

They disadvantage some local businesses

Their lighting is a source of annoyance to someresidents

They hinder emergency vehicles

They waste taxpayers money

They make some motorists more aggressive andimpatient

They detract from the environment

lights, signs, noise


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Problems and difficulties(continued)

They devalue property

They are inappropriate for heavy vehicles

They increase road maintenance costs

They cost people time

Some of these points are not without validity

considerations such as these mean that the

development of traffic calming schemesrequires careful design and sensitiveimplementation


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Speed management

Speed may be managed in two ways Traffic calming

Using physical devices aimed to restrict thespeed of vehicles

Imposition of speed limits

Speeds and safety

There is clear evidence of the effect ofspeed on accident rates

As travel speeds drop, the impact speedsdrop, and collision may be avoided


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Speed management (continued)

Organization for Economic Cooperation andDevelopment has quantified the effect of speed onaccidents and accident severity, based on Swedish

data The percentage drop in accident rates outside built up area

is n times the percentage drop in mean speed

n = 4 for fatal accidents

n= 3 for injury accidents

n= 2 for all accidents

In urban areas, there are two distinct speed relatedaccident problems Injury to drivers and passengers of vehicles where speed is

in excess of posted speed limits


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Speed management (continued)

Injury to vulnerable road users

Pedestrians and bicyclists In many cases the driver is within the speed limit

McLean, et al, 1994 based on a detailed study of 146 fatalpedestrian crashes found that 45 percent of these wouldprobably survived if vehicle struck them with 10 km/h slowerspeed

Speed Limits

Speed limits affect travel speed and therefore shouldaffect accidents specially in urban areas and roads withlimited design standards

There is less clear evidence of affect of speed limits oncrashes in rural areas

Different studies found different results on affect ofincreasing speed limit in rural areas


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Speed limits and travel speeds

Speed limits affect safety only if they affectactual travel speed

The influence of speed limit relies Reasonableness of speed limit as perceived by

driver

On enforcement

Reasonableness of speed limitsIn order to bring about a reduction in mean

speed and speed dispersion, a speed limit shouldbe set at 85 percentile of existing speeds, or at

a lower level ( but not too far below)


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Reasonableness of speed limits(continued)

Road environment factors affecting theperception of driver about the reasonableness ofspeed limit

Alignment Urban or rural environment

Road category

Lane width

Roadside development Traffic density

Sight distance

Parked vehicles

Pedestrians

Day and night vision


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Enforcement

Enforcement would result in a reduction inmean speed and in the spread of speed

This will lead to reduction in crash number andseverity

Automated speed enforcement have beeneffective in reducing speed

Not only on the site where they have been set up

But also in leading to a change in attitudes towardsspeeding

The wide spread use of speed cameras has alreadyproduced a change in driver attitude in Australiawhere portable speed cameras have been in use forseveral years


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Differential speed limits for heavyvehicles

There is evidence that accident rates arerelated to the dispersion or variance ofspeeds of vehicles in the traffic stream

Many different studies reported the chanceof being involved in an accident follows a U-shaped distribution (Figure 13.4 page 335) The minimum occurring when the vehicle is

traveling at about the average speed of traffic,or slightly above

As vehicle speeds move significantly above orbelow the average speed, the probability ofbeing involved in a crash increases dramatically


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Differential speed limits for heavyvehicles (continued)

There is an argument that there should not be aspeed limit differential between heavy vehicles andother road vehicles

Evidence to support this produced in US following theintroduction of nationwide 55 mph car speed limits in1974, the observed speed differential between cars andtrucks was reduced

Radwan and Sinha examined the effect of this on truck

accidents and found There had been reduction in heavy truck crash rates in all

severity types (fatality, injury and PDO) on freeways

On four-lane and two-lane rural highways, the reductionoccurred only for injury crashes

The reductions were attributed to an absolute reduction inspeeds and decreased speed dispersion


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Setting speed limits

There are four types of speed limits

1. General limits

Impose by statute and are applicable to allroads in an area unless signed otherwise

A general urban or rural limit

2. Speed zones

Speed limits applied to a specific roaddepends on the roads design characteristicsand its traffic and land use characteristics

Varying by time of day


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Setting speed limits (continued)

3. Vehicle limits

Apply to specific classes of vehicle

Trucks and buses

4. Driver limits

Apply to specific classes of driver

Learners

Formal management of vehicle speedsusing any of these legally enforceablespeed limits involves

Establishing a balance between safety,

mobility, and amenity for users


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Meeting driver expectation Making speed limits more or less self enforcing

Achieving consistency across the jurisdiction

Ability to deter offenders by appropriate levelsof enforcement

Developing a culture of compliance

Minimizing the cost of signing and enforcement

A systematic approach to setting speed limits in aspeed zoning context is the expert system calledVLIMITS developed by the Australian RoadResearch Board which considers the factors shown

in Table 13.1 page 339


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VLIMITS is used as a basis for a majorstatewide speed limit review in Victoria

The guidelines to use various speed limitsare as follow

Shared zone (6 mph)

Vehicles and pedestrians sharing the same road space Service vehicles in shopping malls

Local traffic area zone (24 mph)

Traffic management works have been undertaken tophysically limit the speed

Local street speed limit (30 mph)

Local access or collector roads with abutting development


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General urban limit (36 mph)

Applies to all urban roads that do not meet thecriteria for higher or lower speed limit

Undivided or divided arterial roads with substantialabutting development

Collector roads with higher standard

25 ft between curbs and carrying above 5000 vpd

Urban (42 mph) Divided roads with direct access to through

roadway

Undivided roads with low traffic volumes, little

or no abutting development


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Urban (48 mph) Divided roads with substantial abutting development

with little or no direct access

Undivided roads with limited amount of abuttingdevelopment

General rural limit (60 mph) Little or no abutting development and widely spaced

intersection

Rural freeway (66 mph) Applies to high standard rural freeways meet the current design standards Have a 30 ft roadside clear zone Accident rate of less than one fatal accident per 1.2 miles

per year

Widely spaced interchanges (greater than 2 miles)


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Devices and techniques formanaging speed and volume

Type of devices and techniques availablefor keeping speed low falls into sixcategories

1. Regulatory devices

Speed limits

Stop and yield signs

No-turn signs

One-way operation

2. Network modifications

Street closure at intersection

Link closure


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Devices and techniques for managingspeed and volume (continued)

Partial street closure

Diagonal closure of intersection

Placement of a barrier diagonally across a cross-

intersection, to create two right-angle bends instead ofan intersection

Closure of median opening

Pedestrian refuge and/or narrow median

3. Devices used at intersections Roundabouts

Resolves priority issues in local streets

Vehicle is required to divert from a straight line

Reduce vehicle speed


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Klyne (1988) established an empirical relationship betweenspeed and path radius

V=6R/SV = 95 percentile speed (km/h) of through

vehicles

R = radius of centerline of vehicle path (meters)

S = sight distance factor (S=1.0 for good sightdistance, up to 1.53 for poor sight distance

This formula suggests that to keep 95 percentile speeds

through an intersection to 30 km/h, the roundaboutgeometry should be such that the path radius developedcan not be greater than 80 feet


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3. Devices relying on horizontal displacement

Chicanes

Feature extending into the roadway from the curb

which referred to as build outs in UK and curbextensions in Australia (Figure 13.12 page 351)

Pinch points

Sometimes referred to as slow points is a short

section of a narrow road Introduces a sudden change in road configuration(Figure 13.13 page 352)

Restructured parking

Rearrangement of parking can be an effective speed

control measure (Figure 14.2 page 371)


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3. Gateways

May be used at the entrance to towns toannounce the start of the build up

environment with devices such as Pinch points

Changes in surface texture and/or color

Road humps

Signing Landscaping features

Planting

Street furniture


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Implementation of devices

Useful guidelines in implementing devicesintroduced based on worldwide researches

Network configuration should be such that theamount of traffic volumes in residential streetsare in the range of 2000-3000 vpd

Wide, long streets with house frontages have apoor safety record and should be avoided

Network discontinuities can be made todiscourage the entry of non-local traffic

Intersections along street within the residentialnetwork should be separated by at least 70 ft


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Implementation of devices(continued)

Action is usually required when 85 percentilespeeds exceed 35 mph

The effect of speed control devices is localized,

which introduce the following relationshipsbetween 85 percentile speeds and spacing

18 mph : 250 ft spacing

24 mph : 330 ft

27 mph : 500 ft 36 mph : 670 ft

Chicanes should not be used when traffic volumeexceeds 600 vph


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Implementation of devices(continued)

Chicanes will constraint vehicle speed to lessthan 20 mph if it is 30-45 ft long

Chicanes and road humps should only used on

straight sections Rumble devices can be problem for bicyclists

and pedestrians

They should not exceed 0.6 inches in height and a gap

of about 30 inches be left between the device and curbto allow passage for bicycles

Road humps lower than 2 inches give virtuallyno slowing effect


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Parking

An important task for traffic management ishow and where to store the vehicles sincethey are spending the majority of their timeat rest

Parked or parking vehicles are particularlyassociated with pedestrian fatalities

Ross Silcock Partnership suggest that planning forparking involves consideration of three factors

The need to maximize access to traffic generatingfacilities

The need to minimize interruption to moving traffic

The need to minimize traffic accidents


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One-way streets

One-way streets tend to be inherently saferthan two way streets Friction from an opposing traffic stream has been

removed Generally one-way operation leads to

Higher speeds and longer trips

Fewer stops

Traffic flow tends to be more orderly More confusing for pedestrians, but on the other

hand conflicts at intersections are reduced

Pedestrian accidents and accidents at low volume

unsignalized intersections are particular problems


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Truck routes

Control of trucks through truck routing is ameans of implementing an urban trafficmanagement scheme

Its application is to pursue amenity objectives, notsafety objectives

The only reference from the safety stand point isto trucks routing of hazardous materials

Certain ideal guidelines from a number of studiesto address the routing for trucks which arecarrying hazardous materials are as follow:

All freeways and control access facilities are likely to be

suitable


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Truck routes (continued)

Routes should be as direct as possible

Routes should be less densely populated

Routes should avoid the centers of populationconcentration, such as schools, shoppingcenters, hospitals and so on

Routes should possibly be free of physicalcharacteristics, such as low overhead

clearance, steep grades, narrow lanes Rail level crossing should be avoided

Crossing over open water supply should beavoided

Documents

N_Traffic Management Chap 13