Beyond Level of Service – Towards a Relative Measurement of Congestion in Planning TransportLauren Walker and Tony FransosVeitch Lister Consulting

What is level of service?A way of translating quantitative traffic performance measures into a quality of service.

Aims to provide qualitative descriptions of:• Traffic flow stability

• Ability of drivers to select desired speeds and to easily manoeuvre

• Incidence of queuing

Adopted by the Highway Capacity Manual and by Austroads

What is level of service?Level of service is a useful performance measure:• Widely accepted

• Simply distils complicated engineering analysis with a highly recognisable measurement scale (A, B, C etc.)

• Uses quantitative performance measures (such as volume-capacity ratios) that are readily available from strategic models as inputs

Drivers perceive and value aspects of congestion in different ways

Problems with level of service

Weinstein (2006) Congestion is subjective, not objective.

Perceptions vary from person to person, cultural context to cultural context.

Arizona DOT

TS 4273 Virginia DOT

Drivers perceive and value aspects of congestion in different ways

Problems with level of service

Papadimitriou et al (2010)Field survey indicated a significant variation in tolerance to various VC ratios from driver to driver

Hostovsky et al (2004)Focus group found that different types of road users valued different aspects of service quality:• Urban commuters value travel time reliability• Rural commuters value manoeuvrability and presence

of HCVs• Commercial vehicle drivers value consistent trip times

and ability to maintain constant speeds

Takes a simplistic view of congestion as a universal sign of network failure

Problems with level of service

Source: Strongtowns.org

Source: The Age

High Street, Northcote

King Street, Newtown

Source: SMH

Taylor (2002)‘Long queues at restaurants…are seen as signs of success’‘Traffic congestion is an inevitable by-product of vibrant, successful cities.’

Doesn’t allow for ‘prioritisation’ of congestion• Primary freeway function is to carry large volumes of

traffic at high speeds for longer distances – minimal interaction with surrounding land uses

• Established inner city areas support other important functions (retail, commercial activity, active/public transport users) as well as conveying traffic

Problems with level of service

Problems with level of service in traffic forecastingStrategic models do not handle delay at intersections well

Link-based VC ratios only account for a small component of variation in travel speeds

Skabardonis (2008)Found that VC ratios account for ~30% of variations in travel speeds on arterial roads – single timing offsets were almost equally as impactful

Problems with level of service in traffic forecastingDifficulties in specifying ‘capacity’ in strategic models

Minderhoud et al (1997) Three types of road capacity :• Design capacity: the maximum volume ‘that may pass

a cross section of a road with a certain probability under predefined road and weather conditions’

• Strategic capacity: ‘the maximum traffic volumes a road section can handle’

• Operational capacity: ‘the actual maximum flow rate’

Problems with level of service in traffic forecastingSpeed-flow curves tend to overestimate traffic volumes under congested conditions

Problems with level of service in traffic forecastingTolerance to congestion will likely grow into the future

Cameron (1996)Level of service measures have not kept pace with changing travel patterns since their initial development in the 1960s. As the public now expects higher levels of congestion, this higher tolerance should be reflected in level of service measures. Clark (2008)Drivers in larger cities, such as in inner-Sydney, have a much higher tolerance to traffic delays than drivers in regional environments. Also, a maximum category of level of service is probably not ‘as bad as it gets’ – a new level of service measures ‘beyond F’ should be considered.

MethodologyTwo-fold approach:1. Identify a VC ratio at which poor level of service is

almost certainly due to excess demand rather than misspecification of capacity

2. Develop simple indices to weight the importance of VC ratio in the evaluation of link performance

ResultsIdentify a VC ratio at which poor level of service is almost certainly due to excess demand rather than misspecification of capacity

Results

Develop simple indices to weight the importance of VC ratio in the evaluation of link performance

Methodology

Index Justification CalculationStrategic importance

Simple measure of the importance of a link in a city's network

Count of appearances of a link in the free flow minimum time paths

Density of human activity

Indication of the number of residents/workers potentially impacted by traffic on each link

Association each link with the average of the population and employment per length of road within an area (SA2)

Amenity Traffic noise generated is a pragmatic proxy for amenity:- higher volumes of traffic reduce ambient

quality- higher speeds increase safety issues- higher numbers of heavy vehicles

produce more noise and emissions- links without crossings and large lanes

numbers decrease the permeability of the surrounding area

Based on approach from Tripathi, Mittal and Ruwali (2012), which is proportional to traffic volume, speed and %HCVs

Modal compatibility

The diversity of speeds and modes that use a link

Shannon's diversity index for speed and volume separately, combined with Shannon’s diversity index range of modes

Results

Less Important

More Important

Results

Better amenity

Worse amenity

Results

Less Dense

More Dense

Results

Fewer modes/travel speeds

More modes/speeds

Develop simple indices to weight the importance of VC ratio in the evaluation of link performance

Methodology

Density of human activity index

Results

• The measure allows transport planners to more easily isolate the relative importance of poor traffic level of service in the context of an entire network, based on key-weighting factors.

Conclusions