traffic management hossein naraghi ce 590 special topics safety january 2003 time spent: 13 hrs

Traffic Management

Hossein Naraghi

CE 590 Special Topics

Safety

January 2003

Time spent: 13 hrs

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 thesefunctions, but in practice there are many roadsthat perform both functions


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


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 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

TF CF/C

T/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: “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)

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


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 many practitioners believe

However, there were several studies which have revealed the positive effects of traffic management schemes


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


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


Improved parking especially for bypass towns• The town can rejuvenate as a business center when

through 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


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 (especially speed humps) in the following terms They discriminating against law-abiding road

users They are dangerous to bicyclists and motor

cyclists, especially when wet

Problems and difficulties (continued)

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 timeSome 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 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

Speed limits affect safety only if they affect actual travel speed

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

driver On enforcement

Reasonableness of speed limits ‘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 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

‘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 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 or below the average speed, the probability of being involved in a crash increases dramatically

Differential speed limits for heavy vehicles (continued)

There is an argument that there should not be a speed limit differential between heavy vehicles and other road vehicles 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 limits1. 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


• 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


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


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) Urban (48 mph)

• Divided roads with substantial abutting development with little or no direct access

• Undivided roads with limited amount of abutting development

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)

Devices and techniques for managing speed and volume

Type of devices and techniques available for keeping speed low falls into six categories

1. Regulatory devices Speed limits Stop and yield signs No-turn signs One-way operation

2. Network modifications Street closure at intersection Link closure

Devices and techniques for managing speed 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 of an 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


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


Channelization To reallocate priority at an intersection

• 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

3. Devices relying on vertical displacement Road humps

• Bump• A pipe half –buried which can potentially damage the vehicle

and cause loss of control


• TRRL or Watts profile hump• Circular in shape, about 2 to 5-inches high and 10-

13 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


3. Devices relying on horizontal displacement Chicanes

• 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)

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)


3. Gateways 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

Useful guidelines in implementing devices introduced based on worldwide researches 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)

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

• 18 mph : 250 ft spacing• 24 mph : 330 ft• 27 mph : 500 ft• 36 mph : 670 ft

Chicanes should not be used when traffic volume exceeds 600 vph

Implementation of devices (continued)

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 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 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

Truck routes Control of trucks through truck routing is a

means of implementing an urban traffic management scheme 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

traffic management hossein naraghi ce 590 special topics safety january 2003 time spent: 13 hrs

Documents

road functions

given road

road configuration

functions of access

local road network

arterial road network

development of road

existing road network