CONGESTION HOTSPOTS
/ 4Report to Transport Panel 22March 2005
Author: Trevor Mason 01992 558615
1. Purpose of Report
This report sets out an assessment of the congestion hotspots in the county as part of the congestion action plan.
2. Background
At the 16th November 2004 Transport Panel a timetable for delivering congestion actions was agreed. One of the first tasks was to develop appropriate strategies to address the problems posed in particular (congestion) hotspots around the county and it was agreed that a report on identified hotspots would be presented to Transport Panel in February 2005.
This paper summarises the initial work undertaken collating different sources of information to start identifying particular congestion problem areas where delays occur on a recurrent basis. However, this paper does not attempt to provide a single definition for congestion, and it must be recognised that solving the issue will depend on how road users themselves determine what is congestion.
Currently the County Council measures the performance of the highway network using Congestion Reference Flows. These are thresholds at which the traffic flow exceeds the capacity of a particular section of road (based on traffic composition, road width, directional split etc). Those sections where the ratio is greater than one are where traffic demand exceeds carriageway capacity and are likely to have serious queuing and congestion on a daily basis.
A plan showing congestion ratios on the motorway and A road network in the county is published annually in the Traffic and Transport Data Report. The ratios are however only based on link capacities and therefore do not include the impact of junctions (which are a major cause of congestion). They are also related to long link sections and it is difficult to pinpoint the actual congestion problem locations (for example the whole of the M25 is defined as having at frequent queuing and congestion during peak hours). There is therefore a need to supplement this information with the identification of more specific congestion points, particularly within the urban areas.
3. Methodology
Various sources of information have been used including anecdotal evidence, information from pre-existing transport studies and transport models and in vehicle
GPS tracking data.
The problem with using different sources of data is that congestion is defined in different ways. For example, hotspots based on anecdotal evidence (ie. where people are aware of delays from their own journeys / home towns) are defined as where people end up queuing in the course of their normal journeys. The delay points identified depend on the route taken and the timing of their journeys. GPS in vehicle data potentially provides a more comprehensive record of journey delays. Congestion in this case is defined by the identification of locations where the vehicle speeds generally drop below a certain threshold on a regular basis. Finally in transport models / studies pinch points are defined as locations where vehicle volume exceeds the road / junction capacity available.
Congestion hotspots could also be classified in different ways, for example a separate analysis of congestion occurring during different time periods (AM and PM Peak) could be undertaken. Alternatively hotspots could be classified according to their cause (e.g. due to lack of highway capacity, parking obstruction, junctions etc).
Further details of the various data sources are set out in Appendix A.
4. Identification of Congestion Hotspots in Hertfordshire
As a starting point all the above sources of information have been pulled together to give an initial indication of the main locations of congestion and delay within the county.
An example of the district tables is shown in Appendix B, and the data is summarised in Figure 1. A full set of the district tables and a colour copy of Figure 1 are available for inspection in the Members’ Room.
These are based on different definitions of ‘congestion’ including locations where vehicle speeds decrease below a certain threshold and locations where vehicles are regularly in a queue. The picture presented is not entirely complete (particularly within the larger urban areas) but this does provide a starting point for further discussion and identification of obvious omissions.
The data will now be fed back into the development of the Area Plans and other appropriate programmes. Further technical work will be undertaken in areas as having a high concentration of congestion hotspots identified through anecdotal evidence.
5. Issues for the Panel to Consider
This paper is for information only.
6. Financial Implications
This paper has no direct financial implications. However, the future development of the congestion action plan may require changes to the current transport programmes.
7. Decision Making Process
This paper is for information only.
Appendix AData Sources
Anecdotal Information
Staff in the Transport Planning & Policy Unit at County Hall were asked to identify locations on their journeys to and from work where they were delayed on a regular basis. Delay was defined as being stuck in stationary / slow moving traffic for longer than a minute, for at least a couple of days during a typical week.
The home locations of staff were scattered around the four corners of Hertfordshire and therefore a reasonable geographical coverage was achieved. The advantage of this method is that it is a quick and easy way to find out the main locations. The disadvantages are that the locations are based on relatively fixed routes to and from work and are also dependent on the time of day travelled. Given many staff at County Hall deliberately choose routes and start times to avoid the worst traffic, the danger is that many of worst delay locations are missed. Another problem is that problems around Hertford which everyone travels through are amplified.
Bus Network Reviews
Anecdotal evidence from discussions with bus operators has also been used by the Passenger Transport Unit to identify operational hot spots for bus services in Watford and surrounding areas as part of the South West Herts Bus Network Review.
The information was obtained with a written consultation (followed up with phone discussions to clarify certain issues). All operators in the area were consulted but it tended to be the larger ones (Arriva, Sovereign and University Bus) who provided the most information. The ‘hotspots’ identified were generally locations where buses experienced problems and delays during the peak period.
A similar process was undertaken in St Albans for the St Albans Transport Plan. South West Herts was the first bus network review this will be followed by a review in the northern area then central Herts then east Herts.
GPS Data (1) – Bus Data
The Passenger Transport Unit at HCC has obtained some sample GPS data from a bus company operating in the south of the County. This is in the form of a listing of times and delays at a series of locations along the route. This data has been interrogated to identify locations along the route where the speed was registered as zero or less than 10 mph on more than one occasion during a particular time period.
The advantage of using this data is that it provides a continuous daily record of journey times and delays. The data available so far however is for a relatively limited time period and therefore could potentially be distorted by short term events such as road works. It is also more time consuming and difficult to interpret data collected remotely. The GPS locations are listed by road names rather than latitude / longitude so on longer sections of road it is difficult to pinpoint the exact location of the delay. Careful thought also needs to be given to the choice of speed threshold and the definition of when a delay is due to congestion (as opposed to pulling into / exiting bus stops / bus stations etc).
There are a number of different GPS systems on the market and these offer the potential for obtaining up-to-date speed and congestion identification on fixed routes. If the opportunity arises, it would be worthwhile obtaining GPS data from a sample of vehicles on the longer distance cross county routes (eg. 724, 300 /301) which would maximise coverage across a number of different areas.
GPS Data (2) – ITIS Data
The DfT have commissioned ITIS Holdings to collect traffic information to collect traffic information in support of congestion targets.
The information is collected by floating vehicle data (FVD). Probe vehicles have GPS tracking units which transmit vehicle location and speed to a central database. The data from probe vehicles is then aggregated to determine the average speed for a given stretch of road.
The resulting data will be provided free of charge to local authorities and potentially will provide a valuable source of journey time and delay data. The data will be provided in Access or SQL format with a GIS map network.
The data is being released on a priority basis to the DfT with release to the metropolitan and large urban authorities first (which have mandatory congesting monitoring in their second Local Transport Plans). The rest of the data is likely to begin in March 2005. The DfT are considering supplying the data on a regional (rather than local authority) basis beginning with the most heavily trafficked areas first. There may be scope for a separate arrangement with the DfT if the data is required more urgently and Gloucestershire and Wiltshire have already received their data.
The data is being provided in a very detailed format comprising average journey times for 15 minute time periods by link. On busy roads the data may be averaged from a number of vehicle records, on quieter roads it could be just one vehicle or there may be no records at all for a particular day. The DfT have calculated that only about 30% of FVD records occur in the more rural areas.
The network covers all motorways, A and B roads. Link length averages about 500m with links generally being defined by junctions with B roads.
The DfT does not intend to produce an annual or monthly summary of data but will instead issue guidance on how to aggregate the data and produce it on a GIS format.
Wiltshire CC is investigating the use of the data for monitoring journey times in Salisbury and on one particular primary route. Consultants have summarised annual information for each link. Ongoing work such as mapping and further statistical analysis is however being done in house.
Even on the primary route (A350) vehicle records were fairly sparse with only 9% of records showing a 15 minute period with more than one vehicle logged. Wiltshire have found with their analysis that the notion of ‘peak time delays’ doesn’t really apply with the spread of journey times in the inter-peak being as much as in the peak.
The ITIS data is a potentially useful source of journey time / delay data. It will however be voluminous and local authorities will be expected to analyse and summarise it themselves. It is likely that away from the motorway / trunk road network and major urban centres that the number of vehicle records will be fairly sparse. The vehicle mix needs careful consideration as some roads may have a high proportion of HGVs / buses which may lower average speeds. The DfT will provide advice on how to eliminate bias from vehicle mix.
Using the data to identify congestion hotspots around the county is likely to be time consuming and laborious. It may therefore be more sensible to use the data to identify the scale of the problem in particular locations. As with the bus GPS data there may also be problems with interpreting the speed data received.
Traffic Information Websites
GPS and floating vehicle data is also provides information for a number of travel information websites. The majority of these are limited to real time information and logs of incidents and roadworks. The Highways Agency traffic information forecaster site however shows predicted journey speeds for a given time period showing different colours for different speed bands. Currently this information is limited to the motorway and trunk road network and is shown at small scale. However, it does give an additional source of information on motorway conditions within the county during the peak periods.
Other Data Sources
Another potential data source includes output from calibrated and validated traffic models. Recently models have recently been updated in St Albans and Bishops Stortford and a model is currently being constructed in Stevenage.
Such models are able to identify junctions within the areas which are over capacity and locations where long queues form. They are a useful additional source of information however, it is crucial that the models are properly validated and calibrated so that they match reality as closely as possible.
An East of England Regional model is also under development. Plots are available which indicate junctions and links where the ratio of volume to capacity is over 0.80 in the Am and inter peak periods for the base year 2001.
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Appendix B
Table 1 – Congestion Hotspots in Broxbourne
Ref / Road /Location
/ Time period / Cause / Evidence from?B1 / A10 SB / Great Cambridge Road Approach to M25 Junction 25, Waltham Cross / Peak periods / Junction M25 / Anecdotal
B2 / Essex Road, Hoddesdon / Peak periods / Level crossing / Anecdotal
Table 2 – Congestion Hotspots in Dacorum
Ref No. / Road /Location
/ Time period / Cause / Evidence from?D1 / M1 / Junction 9-10, Friars Wash / Link capacity issue / 2003 CRF ratio = 0.91
D2 / M1 / Junction 8-9, Hemel Hempstead / Link capacity issue / 2003 CRF ratio = 0.88
D3 / M1 / Junction 7-8, Breakspear / Link capacity issue / 2003 CRF ratio = 1.39
D4 / A414 EB / Breakspear Way (Maylands Avenue to Green Lanes), Hemel Hempstead / Am Peak / Junction with Green Lane / Anecdotal
D5 / A414 WB / Breakspear Way (Green Lanes to Maylands Avenue), Hemel Hempstead / Pm Peak / Junction with Maylands Avenue / Anecdotal
D6 / A4147 SB / Redbourn Road , Maylands Avenue to A414, Hemel Hempstead / Am Peak / Junction with A414. / Anecdotal
D7 / A416 / Kingshill Way, Berkhamsted / Link capacity issue / 2003 CRF ratio = 0.83
D8 / A5 / Watling Street, Markyate / Peak periods / Link capacity issue & junction with M1 / A5183 / 2003 CRF ratios = 0.84 & HA Traffic forecaster site
D9 / A5183 / Dunstable Road, Redbourn / Link capacity issue / 2003 CRF ratio = 0.89
Table 3 – Congestion Hotspots in East Herts
Ref No. / Road /Location
/ Time period / Cause / Evidence from?EH1 / A10 / Rush Green Roundabout, Hertford / Am Peak / Roundabout & difficulty joining A10 from slip roads / Anecdotal
EH2 / A10 WB / London Road, Hertford / Am Peak / Bluecoats roundabout / Anecdotal (& survey info available)
EH3 / A1060 / London Road, Bishops Stortford / Pinch point identified in RPS Masterplanning Study
EH4 / South Street / Station Road / Pinch point identified in RPS Masterplanning Study
EH5 / A1184 SB / London Road, Sawbridgeworth / Am Peak / Anecdotal
EH6 / A119 WB / Ware Road, Hertford / Am Peak / Bluecoats roundabout & Mill Road junctions / Anecdotal (& survey info available)
EH7 / A119 SB / North Road (Bramfield Rd – Hertford North & A414), Hertford / Am Peak / Roundabouts along route & weight of traffic / Anecdotal
EH8 / A120 / Stortford Road, Little Hadham / All day / Traffic signals / Anecdotal
EH9 / A1250 / Hadham Road, Bishops Stortford / ? / Link capacity / CRF Ratio = 2.53
EH10 / A1250 / The Causeway / New Adderley Road / Pinch point identified in RPS Masterplanning
EH11 / A1250 / Hockerhill Street / Dane Street , Bishops Stortford / Junction / Pinch point identified in RPS Masterplanning
EH12 / A1250 / Hockerhill junction, Bishops Stortford / Junction / Pinch point identified in RPS Masterplanning Study
EH13 / A414 EB / Gascoyne Way, Hertford / Peak Periods / County Hall roundabout & Bluecoats / Anecdotal (& survey info available)
EH14 / A414 WB / Gascoyne Way, Hertford (County Hall Rbt to Bluecoats Rbt) / Peak Periods / County Hall roundabout / Anecdotal (& survey info available)
EH15 / A414 / Eastwick Roundabout approaches, Eastwick / Am Peak / Roundabout / Anecdotal
EH16 / A602 / Westmill Road, Paynes Hall / ? / Link capacity / CRF Ratio = 1.12
EH17 / A602 / Woodhall Road, Watton at Stone / Peak periods / Link capacity / CRF Ratio = 0.83
EH18 / A602 / Broadhall Way, Bragbury End (Hooks Cross to Hertford Road) / Link capacity / Anecdotal & CRF Ratio = 1.08
EH19 / B1529 / High Street/ North Street / Bridge Street, Bishops Stortford / Junction / Pinch point identified in RPS Masterplanning Study
Table 4 – Congestion Hotspots in Hertsmere
Ref No. / Road /Location
/ Time period / Cause / Evidence from?H1 / M25 / Jcts21a – 22 Bricket Wood / Peak periods / Link capacity / CRF ratio = 0.84
H2 / M25 / Jcts 23 – 24 (EB) / Pm Peak / Link capacity / Low speeds in HA traffic forecaster
H3 / M25 / Jcts 24-25 Bulls Cross / Peak periods / Link capacity / CRF ratio = 0.91
H4 / A1(M) / Jct 1-2 south Mimms / Peak periods / Link capacity / CRF ratio = 0.85 & low speeds in HA traffic forecaster (pm peak SB)
H5 / A1 / Barnet bypass, Dryham Park / Peak periods / Link capacity / CRF ratio = 0.97
H6 / A1000 / High Street NB, Potters Bar / Peak periods / Route 398 GPS data (consistently low speeds)
H7 / A1000 / Hatfield Road NB, Potters Bar / Am Peak / Route 398 GPS data (consistently low speeds)
H8 / A111 / Southgate Road, Potters Bar / Link capacity / CRF ratio = 0.88
H9 / A5183 / Watling Street, Radlet (ATC site 121) / Peak periods / Link capacity / CRF ratio = 0.83
H10 / B5378 / Shenley Road / Station Road, Borehamwood / Peak periods / SW Herts Bus Review
H11 / B5378 / Shenley Road WB, Borehamwood / Peak periods / Route 398 GPS data (consistently low speeds)
H12 / B556 / Mutton Lane, Potters Bar / Peak periods / Route 398 GPS data (consistently low speeds)
H13 / Darkes Lane SB, Potters Bar / Peak periods / Route 398 GPS data (consistently low speeds)
Table 5 – Congestion Hotspots in North Herts
Ref No. / Road /Location
/ Time period / Cause / Evidence from?NH1 / A1(M) / Jcts 8-9 (SB), Graveley / Am Peak / Blocking back from section to south / Anecdotal evidence & low speeds on HA traffic forecaster
NH2 / A1(M) / Jcts 7-8 (SB), Stevenage / Am Peak / Bottleneck (motorway narrows from 3 lanes to 2) / Anecdotal & low speeds on HA traffic forecaster & CRF ratio = 1.03
NH3 / A1(M) / Jcts 6-7 (SB) Knebworth / Am Peak / Link capacity / Low speeds on HA traffic forecaster & CRF ratio = 1.02
NH4 / A505 / Royston Road WB, Baldock / Peak periods / Royston Road / Whitehorse St signals / Anecdotal (survey information is available)
NH5 / A505 / Offley Road/ Upper Tilehouse St / Parkway Hitchin / Peak periods / Junction capacity (A505 /B655) / Anecdotal
NH6 / A602 / Stevenage Road (NB), Hitchin / PM Peak / Hitchin Hill roundabout / Anecdotal
NH7 / A6141 / Letchworth Gate SB / Am Peak / A1(M) Jct 9 & Blocking back from motorway / Anecdotal
NH8 / B197 / Graveley Road NB / Am Peak / Right turn into North Road blocks back if used as diversionary route to avoid A1(M) / Anecdotal
NH9 / B655 / Pirton Road, Hitchin / Peak periods / Blocking back from A505 /A602 junction / Anecdotal
NH10 / B656 / Hitchin Hill SB / Am Peak / Hitchin Hill roundabout / Anecdotal
NH11 / B656 / High Street, Codicote (SB) / Am Peak / Parking & congestion in village centre / Anecdotal
Table 6– Congestion Hotspots in St Albans