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Tolled Bridges Review: Phase One Report



4. Traffic Trends and Growth

Modelling of the past, current, and future trends of traffic levels, differentiated by time of day/day of week/season (including identification of future developments that may impact on traffic levels)

4.1 Historic Growth Patterns and Traffic Characteristics m

In this section, information about past, current, and future trends of traffic levels is presented and analysed.

4.1.1 Traffic Growth

Toll collection records have been examined for the four bridges since the respective opening dates and the annual figures plotted to show the traffic growth on each bridge. The figures represent two-way flows and have been adjusted as appropriate to take account of periods when no tolls were collected for various reasons. Where bridges are now tolled in one direction only, the flow in the toll direction has been doubled as a proxy. It is however acknowledged that the non-toll direction can be in the order of 3% - 5% higher in such circumstances.

line chart

Generalised observations from this graph are:-

  • Steeper gradient indicates faster growth. Therefore Forth is generally growing faster over the long term.
  • The graph indicates the relative scale of the annual flow over the respective bridges. The Skye flow is small in comparison with the other bridges. The flow over the Forth Bridge is more than double the Tay and Erskine flows.
  • When Erskine Bridge opened to traffic in 1971 its traffic flow was substantially lower than the Tay Bridge but the position has now changed with Erskine now exceeding the Tay flow, particularly as a result of the increases experienced on Erskine since 1997. The flat part of the Tay Bridge curve in the late seventies/ early eighties reflects the opening of Friarton Bridge and completion of the M90 to Perth. This has had the effect of diverting traffic from the Tay Bridge to the Friarton crossing.

4.1.2 Growth Trends

A more detailed examination of traffic growth trends since 1995, when the Skye Bridge was opened to traffic, is presented below. These curves have been drawn using 1995 as the base and the years plotted as a percentage of the base for each bridge respectively. For comparison the growth recorded for major roads in Scotland n is also shown.

line chart

Generalised observations from this graph are:-

  • Skye and Erskine are growing marginally faster than Tay and Forth over the period since 1995, possibly due to a proportionally higher tourism element using these crossings.
  • Tay is closest to the growth curve for major roads in Scotland; with Forth, Erskine and Skye showing higher growth than the average for all major roads in Scotland over this period.
  • The dip in growth experienced generally for all major roads in Scotland due to fuel protests in 2000 is not so clearly reflected in the Bridge flows over the same period.
  • The drop in Erskine traffic in 1996 is due to the oil rig collision. Traffic restrictions were in place on the bridge for several months.
  • Skye dropped in 2001 due to "foot and mouth" outbreak and the resulting drop in tourism. The effect of this is not noticeable on the other bridges where there is a higher proportion of commuter traffic
  • The dip in 1998 on the Forth Road Bridge reflects major roadworks carried out on the bridge during that period.

4.1.3 Seasonal Variation

The monthly values for each bridge have been plotted as a percentage of the respective annual flows, for the year 2003.

line chart

Generalised observations from this graph are:-

  • The Forth, Tay and Erskine curves show similar characteristics, clearly different from Skye, with the peaks in August being less than 10% of the annual flows and the lowest values in February more than 7% of the annual flows, respectively.
  • The Erskine, Forth and Skye bridges show highest flows in July and August but the Tay has a much flatter curve with less traffic variation. Although the Tay peaks in August, the May and September flows are both higher than June and July.
  • The Skye Bridge shows a marked seasonal variation, with the summer peak being almost three times the lowest winter month. The peak month is almost 15% of the annual flow and the lowest month is less than 5%.

4.1.4 Daily Variation

Typical off-peak months have been chosen to represent neutral conditions.

The four histograms below represent each bridge o. Note that the scale is different for each bridge.

bar chart

Generalised observations from these charts are:

  • Tay, Erskine and Forth have comparatively lower flows at the weekends than on weekdays. Forth is the lowest in this respect with both Saturday and Sunday flows about 20% lower than weekday flows.
  • Flows increase progressively over the working week, peaking on Fridays.
  • Skye profile is different to the other bridges with Saturday above the weekday average. However, Sunday is the lowest flow of the week.
  • One-way toll bridges have higher flow in the non-tolled direction.
  • Erskine and Skye have weekend "tide" - Erskine north on Friday and south on Sunday, Skye west on Friday and east on Sunday.
  • On Erskine, more traffic goes north for Monday to Saturday - then the reverse on Sunday.
  • Skye has equal flows in each direction over the whole week.

4.1.5 Weekday Hourly Variation

Dates in neutral months of May and October when tourist traffic is less predominant and weekday commuting patterns are more apparent.

Figures are shown on four histograms below p. Again, note that the scales for each bridge are different. Note that the Tay bridge tolls only in the southbound direction; Forth tolls only northbound; and Erskine and Skye both toll in both directions.

bar chart

Generalised observations from these charts are:

  • Erskine has similar traffic-generating attractions on both sides of the Clyde Estuary.
  • Tay has a large attraction on the Dundee side, promoting "tidal" flow.
  • Forth has an early southbound peak. At 8-9am the peak is fairly even in each direction due to attractions on both sides of the Forth Estuary during that period. The evening peak is predominantly northbound.
  • Skye has a delayed "tidal" morning peak with traffic leaving Skye much later than the other bridges, over an extended period from 8-11. Traffic returns to Skye over a similarly prolonged evening period.
  • Erskine and Tay are very peaky, especially in the morning. Forth shows peak spreading and in-filling of the interpeaks. Consequently, the peaks for the Forth cover an extended period.
  • Skye shows quite a different pattern with the am and pm peaks less predominant and much flatter. There is little interpeak drop indicating relatively more activity during the middle of the day with a lunch time lull between 1-2 pm.

The characteristics of the Skye Bridge indicate a different manner of use, reflecting all-day activity. The other bridges patterns indicate a heavy commuter use and less relative activity during the interpeak.

4.1.6 Traffic Composition q

Due to a lack of availability of consistent data using the same classification system, the data has been drawn from different years. Data is for a typical out of season weekday between the hours of 7 am - 7 pm. Data is from neutral months i.e. April/May/June and September. Note that proportions will change at different times of the year with car proportions increasing for 24 hour flows and at weekends.

pie chart

Generalised observations from these charts are:

  • Comparatively large number of buses on Skye at 4% which includes tourist coaches and local shuttle bus.
  • Proportions are generally very similar during the working day. cars and light goods vehicles are about 90% (89%-92%); other goods vehicles and buses are about 10% (8%-11%).

4.1.7 Comparison with other Estuarial Crossings r

A graphical comparison of the relative daily traffic flows for each of the major estuarial (or river) crossings in Scotland is shown below. The total annual traffic flow in each direction for 2003 has been estimated and divided by the number of days in the year to derive the average daily flow at each location. The tolled bridges are distinguished by blue colouring.

bar chart

All the two-lane single carriageway bridges have flows under 20,000 vehicles per day, except for Kincardine which currently experiences capacity problems at peak periods. Above 60,000 vehicles per day the dual two-lane carriageways also experience congestion at peak periods. Kingston Bridge has dual five-lane carriageways and is clearly in a different category at the centre of a busy urban conurbation, being one of the busiest bridges in Europe. Friarton, Kessock, Erskine and Tay are all dual two-lane carriageways and experience relatively little congestion although the access roads in Dundee experience peak hour congestion to the north of the Tay Bridge.

4.2 Future Trends

The growth trends discussed in section 4.1.2 indicate that traffic on the tolled bridges has grown faster in the period 1995 to 2003 than it has generally grown on major roads in Scotland. Reasons for this are presently unclear. Recognising that national growth projections alone may not adequately reflect future tolled bridge traffic levels, MVA was commissioned to prepare forecasts using the Transport Model for Scotland's Demand and Land Use Models which take account of local population, household and employment projections based on local authority planning data. Due to time constraints within Phase One and the increasingly speculative nature of longer term planning data underpinning TMfS traffic forecasts, MVA was asked only to analyse changes in traffic levels from the base year (2002) to the forecast year (2006) assuming tolls remain unchanged.

As Skye Bridge lies outside the boundary of the TMfS modelled area, Scott Wilson Scotland Ltd was appointed to review the recent traffic trends and to develop a forecast of future traffic growth.

The full MVA report and supporting documents covering Erskine, Forth and Tay Road Bridges are available on request. Key results are summarised below.

The charts below reflect the forecast growth at Clyde, Forth and Tay estuarial crossings. They show that while Friarton, Kincardine and Erskine Bridges are forecast to experience a higher rate of growth in the period 2002-2006 in percentage terms, the greatest additional traffic volumes will occur at Friarton and Forth, with Erskine and Clyde Tunnel experiencing about the same additional traffic numbers.

bar chart

bar chart

4.2.1 Implications for Forth Crossings

As Fife and Falkirk grow in terms of households, population and employment, a higher level of percentage traffic growth over Kincardine Bridge is predicted by 2006. In terms of actual additional average daily traffic, however, the Forth Road Bridge is predicted to attract the majority of additional traffic in 2006. The traffic growth on both Forth crossings has consequences for congestion on the bridges and their approach roads. The additional congestion surrounding the Forth Road Bridge is especially high in the southbound direction where the Bridge is already approaching capacity in 2002. Delays on the A8000 also increase by 2006 with the forecast increases in Forth Road Bridge traffic. The park and ride facility at Ferrytoll is currently being upgraded to hold double the number of cars; this may have an impact on congestion.

4.2.2 Implications for Tay Crossings

As Perth and the surrounding areas grow in terms of households, population and employment, this is reflected by the growth in traffic over Friarton Bridge by 2006. As Friarton Bridge is less congested than other bridge crossings, traffic growth is less restricted. This is in contrast to the Tay Road Bridge where a general decline in population and constraints of congestion result in relatively low growth rates. The congestion analysis indicates an increase in delays over the Tay Road Bridge with Friarton remaining relatively free of congestion. The A90 (Kingsway) passing through Dundee also shows signs of increased congestion in 2006, predominantly due to increased traffic at Friarton and the increase in employment forecast around this area.

4.2.3 Implications for Clyde Crossings

Areas to the north of the Clyde are forecast to grow in terms of households. Although these areas do not show any significant rise in population (apart from further north) this increase along with a rise in car ownership creates an increase in demand over Erskine Bridge. Employment is also set to rise on both sides of the Clyde creating further growth from new households. At present Erskine Bridge has spare capacity in the peak periods, therefore any additional traffic is less restricted, especially compared to the Clyde Tunnel which is presently congested within the peak periods and where any additional demand is suppressed by congestion levels. The majority of additional congestion is shown on approach roads to the north west of Erskine Bridge and to the north of the Clyde Tunnel.

4.2.4 Implications for Skye

The Scott Wilson forecast ( see section 4.2) is based on an analysis of the observed vehicle carryings as recorded by the operators, a comparison with national trends adjusted to reflect conditions generally within Great Britain, Scotland and the Highlands for car-based trips, and an examination of the principal alternative route via the Mallaig-Armadale ferry crossing.

The report concludes that the most likely scenario is that future traffic levels on the bridge will continue to extrapolate from recorded trends whilst reflecting general longer term national growth predictions.

4.3 Traffic Trends and Growth - Key Points

  • Each tolled Bridge has a unique set of traffic characteristics, including differences in bridge user numbers and the time and duration of peak usage. This must be taken into account if and when considering any changes to tolls.
  • Traffic volumes in 2003 (in both directions) were equivalent to a daily average of:

Forth Road Bridge


Erskine Bridge


Tay Road Bridge


Skye Bridge


  • The busiest Scottish estuarial crossing route is Kingston Bridge (untolled). The Forth Road Bridge (tolled) is the second busiest, although only slightly busier than the Clyde Tunnel (untolled). Average daily traffic levels at Erskine and Tay Bridges (tolled) are similar to Kessock and Kincardine Bridges (untolled).
  • Forth, Erskine and Tay profiles show a similar seasonal distribution of traffic, with small peaks over the summer period. Skye shows very marked seasonal variation.
  • Erskine and Skye Bridge traffic flows on a typical off-peak week show weekend 'tides' with a respective increase in northbound and westbound traffic on Fridays, returning over the weekend.
  • During peaks the number of vehicles travelling towards the Forth Road Bridge is greater than the bridge can accommodate. This leads to congestion as drivers queue to cross the Bridge. To avoid this congestion, drivers adjust their time of travel. This is known as 'peak spreading' as the troughs between the morning and evening peaks start to fill up.
  • Skye shows a slower build up of traffic, predominantly leaving Skye throughout the morning and peaking between 10-11am, with a lunchtime lull. This profile indicates a more steady use throughout the day, quite different from the very peaky morning and evening commuter patterns observed on the other toll bridges.
  • Household, population and employment changes forecast for the short term are likely to result in more traffic at Erskine, Forth and Tay Bridges and their main alternative river crossing routes, with further congestion implications for the Forth and Tay Road Bridge and the Clyde Tunnel.