Transcript Traffic Management - Center for Transportation Research

Traffic Management
Time spent: 13 hrs
Hossein Naraghi
CE 590 Special Topics
Safety
January 2003
Road network and functional
hierarchy


Traffic management is a process of
adjusting or adapting the existing road
network to improve traffic operations
without major construction
Traffic management objectives may include

Traffic efficiency
• Road capacity

Improved environment amenity
• Reduced noise
Traffic management objectives
(continued)

Enhanced access
• Better access for particular group of road
users
• Pedestrians
• Bicyclists
• Freight vehicles

Road safety
These objectives may potentially be in
conflict of each other, so priorities may
have to be determined.
Road functions and functional
hierarchy

The use of traffic management to pursue
safety objectives must take place in the
context of a clear view of the functions of a
road network, which is referred to as
functional hierarchy.

Road’s function is the prime determinant of
the management of any given road within
the overall network.
Road functions and functional
hierarchy (continued)

There are essentially two needs from a
road function point of view

The traffic movement function
• The role of roads in providing a means to transfer
people and goods from one place to another; these
roads 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 the
local road network
Ideally each road would perform one of these
functions, but in practice there are many roads
that perform both functions
Road functions and functional
hierarchy (continued)

The only roads that do not have an access
function 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 residential
neighborhoods

Access via frontage roads paralleling an arterial
roads
Road functions and functional
hierarchy (continued)

The roads which carries both functions of
access and mobility, create a major
challenge for traffic management


Tend to have very poor crash record as a result
of their mixed and inherently conflicting
functions
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
Road hierarchy as a network
planning tool


The basis for any traffic management plan is
usually the development of road hierarchy and
agreement to it by various stakeholders.
Main objectives



Prevent residential areas being used by through
traffics
Influence driver behavior to follow planned routes at
moderates speeds, with proper attention being given
to pedestrians and bicyclists
Use physical devices in support of legal regulations in
order to overcome the lack of enforcement for such
measures as speed limits, one way streets and
turning prohibitions.
Road hierarchy as a network
planning tool (continued)

Local roads, which have solely an access
function, will have objectives related to
local amenity and safety
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If road configuration does not allow these
objectives to be met, then there is a need for
some form of treatment
Similarly roads with mixed functions will need
some kind of treatment in order to satisfactory
achieve both safety and mobility
There is also an urban space which is motor
vehicle-free
• Mobility is solely provided by walking and bicycling
Road hierarchy as a network
planning 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 traffic
calming

F/
F
C T/ T
C
C
Gunnarson describes local street (zone C) and two
transition zones (F/C and T/C) as “traffic calming space”
Lessons for new network

Wallwork (1993) said:

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“Traffic calming is a negative reaction to a
problem caused by bad planning, zoning and/or
street design. We need to be proactive in our
approach and learn from the past and others”
This is mainly involve
• Attention to network layout
• Ensure that problems are not built in
• Attention to detail in civic design
• To ensure that an appropriate balance of built and
open spaces and the connection between them are
provided
Lessons for new network
(continued)

Research for the safety aspects of road
network design summarized the key
network planning principles for new
residential areas as follows


Strict differentiation of streets according to their
traffic function leads to safer residential areas
Distribution of traffic into residential area with
multiple access from a ring road is safer than
central distribution
Lessons for new network
(continued)

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Full segregation of vehicle, pedestrians and
bicycle movements is accompanied by very low
accident rates
Cul-de-sac streets are safer than loop streets,
which in turns are safer than ordinary through
streets
Crash rate is minimized where frontage access
is prohibited
Traffic management or traffic
calming


There have been many different
interpretations of traffic calming in different
countries
A useful resolution of this conflict of
interpretation has been proposed by
Brindle

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
Area treatments

Area treatments divided into two
distinct types

Applicable to low density residential
development
• Aim to control speeds which are in excess
of a statutory speed limit of around 30-35
mph

Used in higher density cities
• Aim is to reduce speed to around 20 mph
Area treatments (continued)
Safety effectiveness

Traffic management schemes are usually successful
in reducing
• Vehicle speeds
• Sometimes traffic volumes
• But whether they increase objective safety; as
measured by accident, is open to question

An in depth analysis of accident pattern within case
study area in Sydney concluded that
• Many of traffic management devices used extensively
in local area fail to resolve the dominant types of
accidents occurring on local streets
Area treatments (continued)

The dominant types of accident were



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Cross traffic
Right-through
Rare end
Hit parked vehicles
Local area traffic management schemes may not
be as effective in reducing accidents as many
practitioners believe
However, there were several studies which have
revealed the positive effects of traffic
management schemes
Area treatments (continued)
Environmentally adapted through roads
 This is one which retains its status as
an arterial road
• Traffic efficiency role is curtailed, so other
environmental or amenity goals may also be
realized

There are two typical application
• Bypass towns
• Adaptation of roads still fulfilling their
arterial function
Area treatments (continued)

In either case, treatments typically involve
some or all of the following

A form of gateway treatment using signs and
town 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
Area treatments (continued)

Improved parking especially for bypass towns
• The town can rejuvenate as a business center when
through traffic is removed
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Change traffic furniture like lighting and seating
Enhanced signing
A roundabout at the start and end of the town
• To ensure traffic slows
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Narrower lanes
Change in road surface including color and texture
Additional traffic control measures
• Traffic signal
• Pedestrian crossing
Area treatments (continued)
Problems and difficulties

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There is a negative response from affected
parties which needs to be considered
Brindle (1992) reported a citizen group
opposition to traffic calming (especially
speed humps) in the following terms

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They discriminating against law-abiding road
users
They are dangerous to bicyclists and motor
cyclists, especially when wet
Problems and difficulties
(continued)
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They cause unnecessary wear and tear on motor
vehicles
They disadvantage some local businesses
Their lighting is a source of annoyance to some
residents
They hinder emergency vehicles
They waste taxpayer’s money
They make some motorists more aggressive and
impatient
They detract from the environment
• lights, signs, noise
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 schemes
requires careful design and sensitive
implementation

Speed management

Speed may be managed in two ways

Traffic calming
• Using physical devices aimed to restrict the
speed of vehicles

Imposition of speed limits
Speeds and safety
There is clear evidence of the effect of
speed on accident rates
 As travel speeds drop, the impact speeds
drop, and collision may be avoided

Speed management (continued)

Organization for Economic Cooperation and
Development has quantified the effect of speed on
accidents 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 related
accident problems

Injury to drivers and passengers of vehicles where speed is
in excess of posted speed limits
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 fatal
pedestrian crashes found that 45 percent of these would
probably survived if vehicle struck them with 10 km/h slower
speed
Speed Limits
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
Speed limits affect travel speed and therefore should
affect accidents specially in urban areas and roads with
limited design standards
There is less clear evidence of affect of speed limits on
crashes in rural areas
Different studies found different results on affect of
increasing speed limit in rural areas
Speed limits and travel speeds
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Speed limits affect safety only if they affect
actual travel speed
The influence of speed limit relies
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Reasonableness of speed limit as perceived by
driver
On enforcement
Reasonableness of speed limits
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‘In order to bring about a reduction in mean
speed and speed dispersion, a speed limit should
be set at 85 percentile of existing speeds, or at
a lower level ( but not too far below)’
Reasonableness of speed limits
(continued)

Road environment factors affecting the
perception of driver about the reasonableness of
speed limit
•
•
•
•
•
•
•
•
•
•
Alignment
Urban or rural environment
Road category
Lane width
Roadside development
Traffic density
Sight distance
Parked vehicles
Pedestrians
Day and night vision
Enforcement

Enforcement would result in a reduction in
mean speed and in the spread of speed
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This will lead to reduction in crash number and
severity
Automated speed enforcement have been
effective in reducing speed
• Not only on the site where they have been set up
• But also in leading to a change in attitudes towards
speeding
• The wide spread use of speed cameras has already
produced a change in driver attitude in Australia
where portable speed cameras have been in use for
several years
Differential speed limits for heavy
vehicles
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‘There is evidence that accident rates are
related to the dispersion or variance of
speeds of vehicles in the traffic stream’
Many different studies reported the chance
of being involved in an accident follows a Ushaped 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 or
below the average speed, the probability of
being involved in a crash increases dramatically
Differential speed limits for heavy
vehicles (continued)
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There is an argument that there should not be a
speed limit differential between heavy vehicles and
other road vehicles
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Evidence to support this produced in US following the
introduction of nationwide 55 mph car speed limits in
1974, the observed speed differential between cars and
trucks 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 reduction
occurred only for injury crashes
• The reductions were attributed to an absolute reduction in
speeds and decreased speed dispersion
Setting speed limits

There are four types of speed limits
1.
General limits
 Impose by statute and are applicable to all
roads in an area unless signed otherwise
 A general urban or rural limit
2.
Speed zones
 Speed limits applied to a specific road
depends on the road’s design characteristics
and its traffic and land use characteristics
 Varying by time of day
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 speeds
using any of these legally enforceable
speed limits involves
• Establishing a balance between safety,
mobility, and amenity for users
Setting speed limits (continued)
• Meeting driver expectation
• Making speed limits more or less self enforcing

• Achieving consistency across the jurisdiction
• Ability to deter offenders by appropriate levels
of enforcement
• Developing a culture of compliance
• Minimizing the cost of signing and enforcement
A systematic approach to setting speed limits in a
speed zoning context is the expert system called
VLIMITS developed by the Australian Road
Research Board which considers the factors shown
in Table 13.1 page 339
Setting speed limits (continued)
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
VLIMITS is used as a basis for a major
statewide speed limit review in Victoria
The guidelines to use various speed limits
are 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 to
physically limit the speed

Local street speed limit (30 mph)
• Local access or collector roads with abutting development
Setting speed limits (continued)

General urban limit (36 mph)
• Applies to all urban roads that do not meet the
criteria for higher or lower speed limit
• Undivided or divided arterial roads with substantial
abutting 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
Setting speed limits (continued)
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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 abutting
development
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General rural limit (60 mph)
• Little or no abutting development and widely spaced
intersection
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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)
Devices and techniques for
managing speed and volume
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Type of devices and techniques available
for keeping speed low falls into six
categories
1.
Regulatory devices
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2.
Speed limits
Stop and yield signs
No-turn signs
One-way operation
Network modifications
 Street closure at intersection
 Link closure
Devices and techniques for managing
speed and volume (continued)
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Partial street closure
Diagonal closure of intersection
•
Placement of a barrier diagonally across a crossintersection, to create two right-angle bends instead of
an intersection
Closure of median opening
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Pedestrian refuge and/or narrow median
Devices used at intersections
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3.
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Roundabouts
•
•
•
Resolves priority issues in local streets
Vehicle is required to divert from a straight line
Reduce vehicle speed
Devices and techniques for managing
speed and volume (continued)
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Klyne (1988) established an empirical relationship between
speed and path radius
V=6√R/S
V = 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 sight
distance, 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 roundabout
geometry should be such that the path radius developed
can not be greater than 80 feet
Devices and techniques for managing
speed and volume (continued)
Channelization
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To reallocate priority at an intersection
•
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At t-intersection to give priority to vehicles traveling
around a curve
To deter speeding by motorists on the top of
the t-intersection by requiring them to slow
down to negotiate the device
Devices relying on vertical displacement
3.
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Road humps
•
Bump
•
A pipe half –buried which can potentially damage the vehicle
and cause loss of control
Devices and techniques for managing
speed and volume (continued)
•
TRRL or Watts profile hump
• Circular in shape, about 2 to 5-inches high and 1013 ft in length
• Flat-topped or plateau road hump
• Similar in effect with circular humps but more user
friendly (Figure 13.11 page 349)
• Has straight approach and departure ramp
(typically 1:10 to 1:15) and a flat top,
• Length of flat top varies from 7 to 24 ft, so that
vehicle has both axle on plateau
• Rumble devices
• May be placed transversely across the pavement to
alert the driver to approaching hazard
• Intersections or pedestrian crossing
Devices and techniques for managing
speed and volume (continued)
3.
Devices relying on horizontal displacement
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Chicanes
•
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Pinch points
•
•
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Feature extending into the roadway from the curb
which referred to as “build outs” in UK and “curb
extensions” in Australia (Figure 13.12 page 351)
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)
Devices and techniques for managing
speed and volume (continued)
3.
Gateways
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May be used at the entrance to towns to
announce 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
Implementation of devices
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Useful guidelines in implementing devices
introduced based on worldwide researches
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Network configuration should be such that the
amount of traffic volumes in residential streets
are in the range of 2000-3000 vpd
Wide, long streets with house frontages have a
poor safety record and should be avoided
Network discontinuities can be made to
discourage the entry of non-local traffic
Intersections along street within the residential
network should be separated by at least 70 ft
Implementation of devices
(continued)
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Action is usually required when 85 percentile
speeds exceed 35 mph
The effect of speed control devices is localized,
which introduce the following relationships
between 85 percentile speeds and spacing
•
•
•
•
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18
24
27
36
mph
mph
mph
mph
:
:
:
:
250
330
500
670
ft spacing
ft
ft
ft
Chicanes should not be used when traffic volume
exceeds 600 vph
Implementation of devices
(continued)
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Chicanes will constraint vehicle speed to less
than 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 curb
to allow passage for bicycles

Road humps lower than 2 inches give virtually
no slowing effect
Parking

An important task for traffic management is
how and where to store the vehicles since
they are spending the majority of their time
at rest

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Parked or parking vehicles are particularly
associated with pedestrian fatalities
Ross Silcock Partnership suggest that planning for
parking involves consideration of three factors
• The need to maximize access to traffic generating
facilities
• The need to minimize interruption to moving traffic
• The need to minimize traffic accidents
One-way streets

One-way streets tend to be inherently safer
than two way streets
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Friction from an opposing traffic stream has been
removed
Generally one-way operation leads to
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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
Truck routes

Control of trucks through truck routing is a
means of implementing an urban traffic
management scheme
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Its application is to pursue amenity objectives, not
safety objectives
The only reference from the safety stand point is
to trucks routing of hazardous materials
Certain ideal guidelines from a number of studies
to address the routing for trucks which are
carrying hazardous materials are as follow:
• All freeways and control access facilities are likely to be
suitable
Truck routes (continued)
• Routes should be as direct as possible
• Routes should be less densely populated
• Routes should avoid the centers of population
concentration, such as schools, shopping
centers, hospitals and so on
• Routes should possibly be free of physical
characteristics, such as low overhead
clearance, steep grades, narrow lanes
• Rail level crossing should be avoided
• Crossing over open water supply should be
avoided