CONGESTION SCRUTINY

Congestion Monitoring - National indicator 167

Author: Sue Jackson

Background

The Department for Transport (DfT) has defined National Indicator 167 to measure the level of congestion on local authorities’ road networks. The guidance prescribes that the indicator should relate to average journey time per mile during the morning peak (excluding weekends, bank holidays and school holidays) and be calculated from TrafficMaster data supplied by them (based on journey times collected from in vehicle GPS devices).

The indicator contributes to the evidence about how well an authority is performing its network management duties. This indicator takes advantage of recent technological developments in in-car GPS devices to obtain an unprecedented level of detail about traffic conditions. It is an outcome based indicator, since it directly measures journey times. It can be tracked over time to see how an authority is managing the road network, and how well it is managing the impact of changing demand for travel, and to assess the impact of its planned improvement. Consequently, the indicator enables an evidence-based, targeted approach to tackling congestion.

The baseline indicator is based on data supplied for the 2007/08 academic year. HCC and other authorities have also recently been supplied with data for the 2008/09 academic year. DfT intend to supply the TrafficMaster data on an annual basis with local authorities being required to supply an update of the indicator for the previous academic year in April.

There are three methodologies (prescribed by the Department of Transport) for calculating NI 167, these methods are briefly described below.

Variant 1 - Person journey time per mile during the morning peak on major inbound routes in the larger urban centres.

Variant 2 - Vehicle journey time per mile during the morning peak on major inbound routes in the larger urban centres, weighted by the relative traffic flow on those different routes.

Variant 3 - Vehicle journey time per mile during the morning peak on all major 'A' roads across the local authority.

Hertfordshire County Council in common with the other key authorities in the East of England region has chosen variant 2 as being the most appropriate.

Three options in terms of time period were available, these being 08:00 – 09:00; 07:30 – 09:30 and 07:00 – 10:00. In Hertfordshire the last option was chosen as in many areas the traffic peaks begin before 08:00. Traffic flow data was also only available for hourly segments so this precluded the use of 07:30 – 09:30.

HCC Analysis of Data

The TrafficMaster data supplied by the DfT has been uploaded into a custom designed database called ‘Journey Time Monitoring’ (JTM) developed in-house by the HCC Information Service team. This database contains all the TrafficMaster data provided to date and will be updated with new data as and when it becomes available.

The database allows interrogation of the data and generates reports enabling HCC to undertake analysis of the data in-house rather than having to rely on external consultants.

Variant 2 requires the calculation of journey times in the larger urban centres in Hertfordshire, these have been defined as key centres for change and development and/or key transport nodes in the Regional Spatial Strategy. Bishops Stortford is also included given its proximity to StanstedAirport(a key transport node) and potential development pressures in this area.

The defined towns are as follows:

  • Bishops Stortford
  • Hatfield
  • Hemel Hempstead
  • St Albans
  • Stevenage
  • Watford
  • WelwynGarden City

Variant 2 requires the calculation of journey times on the major inbound routes in these towns. HCC have identified a network of routes into the defined towns. The routes selected were comprised of A and B roads and encompassed the congestion hotspots previously identified in the town.

In the case of St Albans and Bishops Stortford, roads making up the orbital ‘ring road’ system were also included as they are key routes.Figure 1 shows the selected routes and towns.

The database output is the average inbound journey time for each of the routes which is then multiplied by the route length and weighted by the traffic flow on that route to calculate the vehicle minutes per mile. Appendix A gives further details of the calculation methodology.

The DfT only require one figure for the county as a whole. The resulting 2007/08 baseline figure of 2.97 minutes per mile (2 minutes 58 seconds) was supplied in September 2009.

Table 1shows how this was calculated from the individual route segments within the individual towns and time periods.

Table 1 Average Journey times (minutes per mile) for 2007/08 Academic Year

7-8 am / 8-9 am / 9 – 10am / 7-10am
Town / 07/08 / 07/08 / 07/08 / 07/08
Bishops Stortford / 2.62 / 2.60 / 2.26 / 2.51
Hatfield / 2.42 / 2.35 / 2.11 / 2.30
Hemel Hempstead / 3.24 / 2.77 / 2.33 / 2.77
St Albans / 4.27 / 3.75 / 2.93 / 3.67
Stevenage / 2.83 / 2.72 / 2.20 / 2.60
Watford / 3.52 / 3.34 / 3.09 / 3.31
WelwynGarden / 2.85 / 2.64 / 2.15 / 2.59
County total / 3.29 / 3.02 / 2.56 / 2.97
(2:58)

The resulting speeds are mapped in Figure 2.

Comparison with other authorities

In February the DfT published the baseline journey time figures for the other authorities using Variant 2. These are shown as Table B.1 in Appendix B (with figures converted into minutes and seconds).

Hertfordshire has the second quickest journey times in the East of England region (after Essex) and has faster journey times than most other authorities in the list. Journey times are comparable with those calculated in Poole and Bournemouth.

Average journey times will vary depending on the time period selected and the routes selected and therefore the DfT emphasises that direct comparisons should be treated with caution. HCC has contacted the other local authorities in the area to determine the selection criteria used. The criteria of those who responded are shown in Appendix B (Table B.2).

The key difference between Hertfordshire and the other authorities is the choice of time period. Hertfordshire defines the AM peak as the full three hours (0700-1000) whereas those authorities who have responded used a shorter peak (0730-0930 or 0800-0900). Table 1 above indicates that the journey time in Hertfordshire becomes considerably faster between 0900 and 1000 and this has decreased the overall average journey times. An average journey time based on the other two hours would be over 3 minutes which would be more in line with other authorities.

A second factor is the choice of journey time routes. Essex’s low journey times can be explained by the inclusion of long sections of dual carriageway routes with high speed limits, whereas Cambridgeshire includes single carriageway routes in urban areas subject to 30 / 40mph speed limits. Hertfordshire’s routes include some sections where the national speed limit is applicable (eg A120 in Bishops Stortford) and sections of dual carriageway in the newer towns Hemel Hempstead, Hatfield and Stevenage which have higher journey speeds.

2008/09 Data Update

The DfT supplied the TrafficMaster data for the 08/09 academic year at the end of 2009. This data has been fed into HCC’s database and analysed to produce an updated journey time figure which was submitted to the DfT in April 2009. Table 2 shows the updated data.

During the AM peak in general all towns except Stevenage experienced a decrease in journey times between 07/08 and 08/09. Overall there has been a reduction in AM journey times by 3.3% between 07/08 and 08/09.

This is in line with traffic flow figures which show an overall decrease in traffic levels across the county of around 1.6% between 2008 and 2009.

The decreases were most marked during the early part of the AM peak between 7-8am, particularly in Bishops Stortford, Hemel and St Albans. Individual routes and link sections within these towns have been compared. Some key findings are as follows:

Bishops Stortford –Improvements in journey time on approaches to town centre (Adderly Road / Hockerill junction area).

Hemel Hempstead – general improvement of speeds could be related to completion of M1 widening works and reduction of traffic on local roads.

St Albans – Improvements in journey time on approaches to key junctions on Batchwood Drive / Beechwood Avenue and on approach to the Peahen junction.

It is possible that general reductions in traffic flow have reduced congestion on the approaches to these key junctions.

Table 2 2008 /09 Average Journey times in Hertfordshire

Ave journey time Minutes /mile
Town / 08/09 / % change from 07/08 / 08/09 / % change from 07/08 / 08/09 / % change from 07/08 / Total 7-10am
08/09 % change
Bishops Stortford / 2.40 / -8.5% / 2.62 / 0.5% / 2.25 / -0.8% / 2.45 / -2.6%
Hatfield / 2.32 / -3.8% / 2.34 / -0.2% / 2.11 / -0.1% / 2.27 / -1.3%
Hemel Hempstead / 2.88 / -11.2% / 2.86 / 3.1% / 2.27 / -2.3% / 2.67 / -3.5%
St Albans / 3.71 / -13.2% / 3.65 / -2.7% / 2.76 / -5.7% / 3.40 / -7.3%
Stevenage / 2.79 / -1.3% / 2.86 / 5.3% / 2.20 / -0.3% / 2.65 / 2.1%
Watford / 3.57 / 1.5% / 3.29 / -1.2% / 2.70 / -12.5% / 3.19 / -3.6%
WelwynGarden / 2.81 / -1.7% / 2.74 / 3.7% / 2.16 / 0.8% / 2.58 / -0.3%
County total / 3.09 / -5.9% / 3.03 / +1% / 2.43 / -3.6% / 2.87 / -3.3%

The DfT has recently published the 08/09 figures1 and % change for the local authorities using the variant 2 data. These are summarised Appendix B.

Hertfordshire’s reduction in journey times is contrary to the pattern in the rest of the East of England but a number of the other southern counties including Hampshire Surrey and Oxfordshire also experienced a decrease.

The DfT Bulletin also states that there was a decrease of 7.3% in average vehicle delay on the slowest 10% of journeys on interurban (Highways Agency Roads) between 2007 and 2008. There was also a decrease of over 2% in journey times in the 10 major urban areas between 07/08 and 08/09.

Hertfordshire’s decrease in journey times is therefore in line with these wider trends[1]

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Appendix A – Calculation Methodology

Calculation of NI167 indicator for DfT

The indicator supplied is the average vehicle journey time per mile. This is defined as the average time it takes a non stopping vehicle to travel one mile on specified routes during the morning peak, excluding school holidaysand includes cars, light vans and HGVs.

This indicator is calculated in a number of stages as follows:

1.Journey time routes defined in GIS are uploaded into the Journey Time Monitoring Database (JTM).

  1. User specifies time period of interest, hours of interest and route in JTM. Output produced gives average journey times (in seconds) and link lengths for individual route segments and journey routes as a whole.
  1. Data is input into Excel spreadsheets. The total journey time in minutes is calculated by taking the route totals in seconds and dividing by sixty as follows:

Total vehicle minutes = Total journey time = ∑(flow * SJT)

Where SJT is segment journey time and ∑ sums across all segments and peak hours

  1. Then the total vehicle miles per route are calculated by multiplying the traffic flow on the route by the total route length as follows:

Total vehicle miles = ∑(flow * segment length)

Where ∑ sums across all segments and peak hours

  1. The total town vehicle minutes are then divided by the total town vehicle miles to give the final indicator for one town.

Annual average minutes per mile = Total journey time / Total vehicle miles

  1. These are then summed to give the county totals which are supplied to the DfT

Appendix B Published NI167 Data

Table B.1 National indicator 167 values for local authorities covered by variant 2
Vehicle journey time during the morning peak1 on key monitored routes: 2007-08
Average vehicle journey times
Local Authority / 2007-08
East Midlands
Derby / 03:26
East of England
Cambridgeshire / 04:08
Essex / 02:11
Hertfordshire / 02:58
Luton / 03:37
Norfolk / 03:50
Southend-on-Sea / 03:51
North East
Durham / 04:06
North West
Blackburn and Darwen / 04:22
Blackpool / 03:20
Lancashire / 04:11
Warrington / 03:12
South East
Brighton and Hove / 03:45
Hampshire / 03:29
Medway / 04:00
Oxfordshire / 04:17
Reading / 04:20
Surrey / 03:40
West Sussex / 03:01
South West
Bournemouth / 02:51
Devon / 04:20
Dorset / 03:05
Poole / 02:51
West Midlands
Warwickshire / 03:55
Yorkshire & The Humber
Kingston upon Hull / 04:46
North East Lincolnshire / 03:21
North Yorkshire / 03:18
York / 03:24
1. The morning peak is defined by each local authority

Reproduced from Table 8 published in Road Traffic and Congestion in Great Britain Q1 2010, DfT

Table B.2 Selection criteria used by other authorities

Council / Variant 2 Used? / Time Period of Analysis / Towns and Routes Selected / Map?
Luton Borough Council / Yes / 0730-0930 / Only one urban centre was used, the town centre, using 6 inbound routes (5 radial and 1 orbital) / No
Cambridgeshire County Council / Yes / 0730-0930 / Includes 9 congested radial routes into centre of Cambridge, 2 routes in St Neots, 4 in Huntingdon, 2 in March, 2 in Wisbech and 2 short sections approaching Ely / Yes
Essex County Council / Yes / 0730-0930 / Harlow, Brentwood, Basildon, Canvey, Chelmsford, Witham and Colchester towns have all been included. Routes include dual carriageway sections of the A12 through Brentwood and Chelmsford and the A121 between the M25 and A120 / Yes
Norfolk County Council / Yes / 0800-0900 / 12 radial routes and 2 orbital routes in Norwich, main north south route (A143 / A149) and key approach roads in Great Yarmouth, A17, A1078 and A1076 in Kings Lyn. / Yes
Surrey County Council / Yes / 0730-0930 / 8 key radial routes in Guildford, A23 & A217 plus 2 EW routes in Redhill / Reigate, 6 radial routes into Woking / Yes

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[1] Road Traffic and Congestion in Great Britain Q1 2010 DfT