Introduction
An uncomfortable position
A wicked problem
A challenge to the forecast-led transport planning paradigm
A case-study approach
Opening out
Overview of the road traffic forecasting approach
Modification of the approach
Comparing forecast ranges across NRTF reports
Report year | Forecast period | Forecast range (% change in total traffic) | Degree of fana (forecast range/forecast period) base 1989 = 100 | |
---|---|---|---|---|
1989 | 1988–2025 | 83–142 | 100 | |
1997 | 1996–2031 | 36–84 | 86 | |
2007 | 2003–2025 | 20–39 | 54 | |
2008 | 2003–2025 | 22–40 | 51 | |
2009 | 2003–2035 | 31–50 | 37 | |
2011 | 2010–2035 | 34–55 | 28 | |
2013 | 2010–2040 | 23–67 | 91 | |
2015 | 2010–2040 | 19–55 | 75 | |
2018 | 2015–2050 | 17–51 | 61 |
Report year | Forecast population change (%) | Rate of population changea (population change/forecast period)-base 1989 = 100 | |
---|---|---|---|
1989 | 4.9 | 100 | |
1997 | 3.3 | 71 | |
2007 | 8.5 | 292 | |
2008 | 14.5 (+ low migration) | 498 | |
2009 | 21 | 496 | |
2011 | 18 | 544 | |
2013 | 20 (low 10; high 30) | 503 | |
2015 | 19 | 478 | |
2018 | 19 (low 13; high 24) | 410 |
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2013 forecasts—the scenarios at the edges of the forecast fan were ‘low population + low GDP + high oil price’ and ‘high population + high GDP + low oil price’. Population uncertainty thus played a ‘full’ part in influencing the forecast fan (contrary to the previous reports2).
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2015 forecasts—no uncertainty in population was considered in the set of scenarios forming the forecast fan.
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2018 forecasts—uncertainty in population is considered through two scenarios but not in combination with uncertainty in GDP and oil price as it was in the 2013 exercise. These two scenarios do not define the high and the low end of the forecast fan as they did in 2013. These instead are defined by: (1) a scenario that considers ongoing decline in trip rate along with ongoing decline in young people’s driving licence holding; and (2) a scenario that considers a strong shift to zero emission vehicles (see further below). Neither of these scenarios consider, as far as we are aware, compound effects of population uncertainty (high/low migration).
Reaching beyond the traditional drivers of road traffic
When is opening out sufficient?
Closing down
A disconnect between opening out and closing down
A regressive approach to scheme appraisal?
Opening up
Time for change
Policy challenges and professional appetite
A plausible new approach
Changing norms
Confronting inertia
Research challenges
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Understanding a changing world—Whilst we identify above that there is considerable professional uncertainty about likely directions of future travel demand shifts, this is also a research gap (Marsden et al. 2018; Shaheen et al. 2018). The plausibility of different combinations of social change, transport technology change and behavioural adaptation (to both) is not well-understood. A deeper understanding of the extent to which different futures could unfold, given existing land-uses, cohort effects, technology transition periods and evidence of pace of change to date should be developed.
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Understanding the decision makers’ perspective—It is well established that decision-makers reduce the complexity of their tasks by focussing on significant cues (Shanteau 1992). However, we find little evidence on what those cues are or how they relate to the cognitive limitations of decision-makers, particularly in the transportation assessment context (see Nellthorp and Mackie 2000 as an exception). Even were such limits to be established or understood we see no reason why step-wise approaches to decision-support could not be developed which reduced the risk in cognitive overload (see Dodgson et al. 2009). Rather than presuming what decision-makers can cope with and working back from there to the current forecasting and assessment approach, this should be empirically and iteratively tested.
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Appropriate scrutiny of different categories of intervention—How uncertainty is handled and the level of scrutiny applied may vary according to different stages in an appraisal process but also according to different categories of project or policy. This concerns proportionality, particularly in the face of limited analytical resource. It would be helpful to empirically examine how different categories of schemes and interventions might become distinguishable and in turn guide which approach(es) would offer proportionate scrutiny in each category. This might differ by place (e.g. core urban areas versus inter-urban corridors) as well as by intervention (e.g. train electrification of existing lines versus constructing new lines).
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Adaptive planning—Uncertainty is not a steady-state phenomenon. While the alternative approach set out in this paper can be better suited to handling deep uncertainty than the forecast-led approach, it is at risk of being static in its application. In other words, as the world continues to evolve (in some ways potentially profoundly and rapidly) the knowledge we have about trends and views about the nature of future uncertainty will also change. A candidate policy or investment may presently be judged to strike a good balance between risk and yield; yet this may change. As such, there is an important role for ongoing monitoring and review of policies and investments. It is not clear the extent to which current or evolving practice is accounting for this or has the capacity to do so. This merits further attention (see Roelich and Giesekam 2019 and Marchau et al. 2019).