FOREWORD Introduction How Have We Carried Out The Measurements? Congestion Indicators Changes to the Indicators NATIONAL TRUNK ROAD CONGESTION INDICATORS Glossary of Terms Technical Annex
FOREWORD
The Scottish Government has a single overarching purpose; to focus Government and public services on creating a more successful country with opportunities for all of Scotland to flourish through increasing sustainable economic growth. Transport is a key enabler in delivering a successful, dynamic nation; as we all rely on transport for the movement of goods, services and people as part of our daily lives.
The Government Economic Strategy was launched at the end of 2007 alongside the National Performance Framework contained within the Scottish Budget Spending Review 2007, which underpins the Economic Strategy. In the National Performance Framework there is an indicator to "Reduce the proportion of driver journeys delayed due to traffic congestion" which emphasises the importance of reducing congestion.
This report is a re-publication of the 2nd report in a series of reports setting out levels of congestion on 44 monitored sections of the trunk road network in 2004. It is important to note that this report does not cover the whole trunk road network. It focuses on those sections of trunk road which presently experience congestion or are likely to experience congestion over the coming years. It does not consider those sections of the network that are currently considered to be operating freely within their capacity.
We will use this and subsequent reports in the series to provide better information on trends that will inform policy development and help guide the Government’s long-term response to congestion; and ultimately improve the performance of Scotland’s trunk road network.
This report was produced by the Network Operations’ Scottish Roads Traffic Database staff. A range of relevant congestion indicators were developed as part of the inaugural report in 2003. Since the publication of this initial and subsequent 2004 report, the Government has revisited the original indicators to ensure the information provided in these reports remains useful. This process has seen several of the indicators removed or replaced by new measures.
In light of these changes this is the re-publication of the 2nd report in the series, the 2004 data, so that the 2004, 2005 and 2006 data reports are now consistent in the use of the new suite of indicators.
Introduction
What is congestion? If you canvassed 10 different people you would probably get 10 different answers. As outlined in the original 2003 report, it has been decided that the main measure of congestion in people’s minds is that the speed of travel on all or part of a journey is less than anticipated. This is translated into the journey taking longer than anticipated. Therefore the additional time taken to complete a journey is what is important to the individual.
Much congestion is recurrent and travellers factor additional time into their journeys to take account of this. However incidents on the network can often worsen congestion and increase journey time further. It is important therefore to consider how often unexpected delays occur. This is know asjourney time reliability.
The average time lost by a single vehicle over one kilometre of the route is also a useful indicator and takes into account the number of vehicles on the route. This is known as lost time per vehicle kilometre.
The spread and impact of the number of vehicles experiencing congestion is important and we have a local trunk road congestion indicator which separates the congestion experienced into three bands of mild, serious, and severe congestion.
Although it is attractive to try to have single indicators to cover the whole country this does not give a flavour of local situations. There is a need to provide local congestion measurements.
How Have We Carried Out The Measurements?
Congestion can be measured in many ways. The method that we have chosen uses traffic volume and speed measurements taken from automatic traffic counters located at fixed traffic monitoring sites throughout the Scottish trunk road network. These counters collect data on the speed, distance travelled, volume, and composition of traffic on the monitored routes. This is then stored in 15 minute intervals, allowing us to see how these measures vary through the day, week, month and year. The speed data for each section of the road covered by a monitoring site is then calibrated by using data collected from what are called "floating vehicle" surveys. In these surveys, vehicles actually drive the routes at speeds which are representative of the traffic flow in which they are travelling (by balancing the numbers of vehicles which they overtake and the number that overtake them) and measure the speed and times.
Congestion Indicators
Given the variety of input data available, it would be possible to develop a wide range of congestion indicators. However, in developing the congestion data reports, we have attempted to develop indicators that are meaningful, intuitive and relatively simple to understand.
In developing our congestion indicators, we have focused on two main types of indicator – those which can give anational picture of congestion, ie those that can provide us with a general insight into how congestion is impacting on traffic using Scotland’s trunk road network, and those which can give a local indication of congestion, providing us with a more fine scale understanding of how congestion is affecting individual routes in the network.
Changes to the Indicators
The previous 2003 and original 2004 reports used congestion indicators NCI-3 (a National indicator) and LCI-3 (a Local indicator) which were developed to try to provide an indication of the "cost to the economy" which is attributable to congestion on the routes in question. However, subsequent to the publication of the 2004 Data Report, a decision was taken in 2007 to no longer use the cost to the economy indicator due to increasing reservations as to its robustness.
In the past we had also used local congestion indicators LCI-5 (which measured the total time lost per kilometre per day) and LCI-7 (which was an annual average daily congestion index). In order to improve the clarity of the analysis and to avoid any unnecessary confusion, we have decided to no longer use these two indicators. However, to maintain consistency and to assist with any future comparisons, we have decided to continue using the established nomenclature for the indicators. As a result, you will notice these reports will refer to LCI-1,2, 4 and 6, but exclude any specific reference to LCI-3, LCI-5 and LCI-7.
For further detailed explanations of the methodologies and current indicators please refer to the technical annex at the end of this report.
NATIONAL TRUNK ROAD CONGESTION INDICATORS
The indicators for 2004 are as follows:
National Trunk Road Congestion Indicator 1 (NCI-1) – Additional Travel Time per Annum
7,930,000 Hours |
This indicator is the additional travel time for the year which is attributable to traffic congestion.
It is important to note that due to the changes in network coverage this figure cannot be directly compared to figures published in earlier reports. However, it is useful in that it provides us with a general idea of the significance of congestion to those parties using the monitored routes in the network.
National Trunk Road Congestion Indicator 2 (NCI-2) – Average Time Lost per Vehicle Kilometre
4.89 seconds |
This is the average time lost per vehicle kilometre averaged over the year. This is the definition of delay used by the United Kingdom Department for Transport. It is also used as a general measure of congestion by other roads authorities throughout the world.
The rest of this report sets out the level of congestion in terms of local congestion indicators and gives additional useful route information.
Glossary of Terms
The following terms are used throughout this report. Some explanation and further details are required.
1 Location map
shows the general coverage of each of the two directional
routes included in the page. Note that the start and end of one
direction (eg EB) may not coincide exactly with the end and start
of the opposite direction (eg WB).
2 Route Length (km)
shows the length in kilometres between the start and the end of
the coverage. In most cases the two directional lengths will be
slightly different. Where there are significant differences these
are explained in the note at the foot of the page.
3 Free Flow Speed (kph)
shows the average speed, in kilometres per hour, for this
route when there is no congestion.
4 Free Flow Journey Time (minutes)
is simply a function of the Free Flow Speed and the route
length.
5 Average Daily Actual Journey Time (minutes)
is a function of the Free Flow Journey Time for the route, the
Average Lost Time per Vehicle Kilometre for the route and the route
length.
6 Average Annual Daily Flow
the average daily flow of vehicles for the entire length of the
route.
7 LCI 1 - Lost Time per annum (hours)
the time lost when compared to free flow speed by all vehicles
over the entire length of the route in a year.
8 LCI 2 - Lost Time per Vehicle Kilometre
the average time lost by a single vehicle over one kilometre of
the route.
9 Average Lost Time for route (mins)
is simply a function of LCI 2 and the length of the route and
is the average time a single vehicle would lose over the route.
10 LCI 4 - Journey Time Reliability
a measure of how regular is the congestion at any particular
time of the day, averaged over the entire day, based on a
definition of "Reliable" as being +/- 15% of average journey
time.
11 LCI 6 shows the number of vehicles and the duration (to the nearest quarter hour) of ten different degrees of congestion.
12 LCI 6 - Equivalent Max Journey Times
this is the journey time associated with the upper end of the
Speed Drop band and is calculated from the Speed Drop, the Free
Flow Speed and the route length.
Note The LCI values shown in this report are subset of the indices available.
Area Name |
Route Description |
Aberdeen |
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Aberdeen |
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Aberdeen |
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Dundee |
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Dundee |
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Perth |
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Perth |
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Forth Bridge Approaches |
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Kincardine Bridge Approaches |
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Erskine Bridge |
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Edinburgh |
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Edinburgh |
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Edinburgh |
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Glasgow |
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Glasgow |
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Glasgow |
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Glasgow |
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Glasgow |
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Glasgow-Edinburgh |
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Ayrshire |
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Ayrshire |
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Ayrshire |
Technical Annex
Description of General Methodology
A very brief explanation of the methodology:
Speed = Distance/Time and therefore,Time = Distance/Speed
If we restrict initial calculations to distances of 1 kilometre then, Time = 1/Speed
If we measure the Free Flow Speed (FFS) – i.e. the speed at which vehicles will travel over a site when not restrained by other vehicles – then we can calculate congestion, as an increase in journey time over the length of route associated with that site in any single 15 minute time period as:
Lost time per vehicle kilometre (LTVK) = 1/V – 1/FFS
(where "V" = the average speed of all vehicles in that time period).
If all sites were located within quite short sections of road and no priority junctions were involved then the above equation would be enough. However, even on the trunk road network in Scotland there are junctions where trunk road travellers must give way to other road users.
Consider Sheriffhall Junction on the A720 City Bypass. A simple traffic counting site located on the A720 to the east of that junction will pick up the congestion due to decreased speeds along that section of the route but will not pick up the congestion due to queuing at the junction nor decelerating on approach to the junction nor accelerating away from the junction.
A series of floating vehicle surveys was carried out, resulting in something of the order of 260 trips across each traffic counter site and the data used to calculate actual travel times between link ends (a link is the section of road associated with each site) as well as contemporary speed at the location of the sites. A regression analysis then produced a calibration factor "m" which was used to adjust the LTVK equation above as follows:
LTVK = 1/mV – 1/mFFS
Most other congestion indices calculated for a single 15 minute period for a single site are basic summations of LTVK. Journey Time Reliability is different and is explained further on page 30.
The relevant site-based congestion numbers are then summated, and averaged, for all sites in a route.
Congestion Indicators
Given the variety of input data, including speed, time, distance, volume of traffic, composition of traffic, time of day, day of week and time of year it would be possible to develop a great many indicators. What we have attempted to do is to develop indicators which are relatively simple to understand and which are intuitive to use.
There are two main types of indicator – those which can give a national picture and which can be used to track national trends over time and those which can give a local indication of congestion. The following explains the indicators chosen to be used at national and local levels.
National Trunk Road Indicators
National Trunk Road Congestion Indicator 1 (NCI-1) – Additional Travel Time per Annum
This indicator is the additional travel time for the year which is attributable to traffic congestion.
If over a period of one hour the average time taken to cover a specific route increased by 10 minutes over the time it would have taken under Free Flow Speed 1 conditions and 1200 vehicles went through in that hour, then the total additional time would have been 1200 x 10 = 12,000 minutes (200 hours). Such figures can be aggregated to provide annual figures which can be used for a route. When each route is aggregated up, a national figure for the Additional Travel Time due to congestion can be produced. Daily, weekly and monthly figures could also be produced.
National Trunk Road Congestion Indicator 2 (NCI-2) – Average Time Lost per Vehicle Kilometre
This is the time lost per vehicle kilometre averaged over the year. This is the definition of delay used by the United Kingdom Department for Transport. It is also used by other roads authorities throughout the world.
Average Time Lost per Vehicle Kilometre is effectively "Additional Travel Time" (NCI-1) divided by the "total volume of traffic" (expressed as total vehicle kilometres travelled). This gives the average delay encountered by a vehicle travelling one kilometre. With careful application this figure could be used to benchmark congestion levels against other countries.
Local Trunk Road Indicators
For the purpose of this trunk road congestion monitoring exercise, the network has been broken down into 10 areas within which there are 44 distinct routes and we have developed Local Trunk Road Indicators as follows:
Local Trunk Road Congestion Indicator 1 (LCI-1) – Additional Travel Time per Annum
This indicator is the additional travel time for the year which is attributable to traffic congestion on the route in question.
Additional Traffic Time within a route will be measured using the same methodology as for NCI-1. Daily, weekly and monthly figures could also be produced.
Local Trunk Road Congestion Indicator 2 (LCI-2) – Average Time Lost per Vehicle Kilometre
This is the time lost per vehicle kilometre averaged over the year for the route in question. Average Time Lost per Vehicle Kilometre averaged over the year is measured using the same methodology as for NCI 2.
Local Trunk Road Congestion Indicator 4 (LCI-4) – Journey Time Reliability
This indicator has been developed to give an indication as to how often the time taken to complete a journey is significantly more than the norm for the route and time period in question.
LCI-4 measures the percentage of journeys over each complete route and within each 15 minute time period which take more than 15% longer to complete than the annual average journey time for that same period. If a particular journey takes on average 30 minutes then any journey that takes longer than 34 minutes 30 seconds (30 x 1.15) will be counted. All journeys over this length of time will be counted and compared against the total number of journeys.
Example calculation if a route has 7,000,000 journeys in a year and 500,000 take longer than 115% of the annual average time then
Journey Time Reliability = |
7,000,000 – 500,000 |
x 100% = 92.86% |
7,000,000 |
Local Trunk Road Congestion Indicator 6 (LCI-6) – Three Congestion Bands
This methodology separates the congestion experienced into three bands of Mild, Serious and Severe. This indicator is designed to try to give some measure of the impact of congestion. The definitions chosen are as follows:
Three Congestion Band Comparison
In other words, if the average speed in any 15 minute period is below the free flow speed by more than 10% but not exceeding 25%, then that 15 minute period is defined as exhibiting mild congestion, and so on. This allows the impact of congestion to be tracked over time. An increase in mild congestion might be deemed to be acceptable with small increases in Additional Travel Time being spread over many journeys. However, an increase in severe congestion with significant increases in Additional Travel Time might be deemed less acceptable.
The three bands described above have also been sub-divided further into 10 bands for more detailed analysis, as follows:
Ten Congestion Band Comparisons
‘Mild’ congestion is now sub-divided into Bands 2, 3 and 4; ‘Serious’ into Bands 5, 6, 7, 8 and 9; while ‘Severe’ is in Band 10.
We have calculated and illustrated in this report the equivalent maximum journey time associated with the upper end of the Speed Drop band and is calculated from the Speed Drop, the Free Flow Speed and the route length. This information provides the reader with some additional context.
Reasons why north/south or east/west route lengths may vary
It will be observed that the distance in the opposite direction for a number of the routes differs. There are a number of reasons for this usually relating to individual route/junction layouts, particularly where slip roads are involved or large roundabouts. On occasions one-way streets may influence the respective route lengths for opposite directions. Note also that the start and end of one direction (e.g. Eastbound) may not coincide exactly with the end and start of the opposite direction (e.g. Westbound).