Is Transport Assessment Fit for Vision-Led Planning?

By Colin Black and Joey Hill

The shift toward vision-led planning represents a deliberate move away from predict-and-provide and toward a more outcomes-focused approach to transport and place. While the ambition is widely supported, the key issue now is how this translates into practice and whether the tools we currently rely on are capable of supporting that transition.

With the forthcoming Planning Practice Guidance (PPG) on vision-led transport planning expected later this year, there is a clear opportunity for change. However, there is also uncertainty. To what extent will the new guidance fundamentally reshape transport assessment practice, and to what extent will it sit alongside, and potentially be constrained by, established methodologies embedded in the modus operandi of development planning?

This question is particularly important given growing concern about how existing approaches perform in practice. There is increasing evidence that conventional Transport Assessments (TA) can significantly overestimate vehicular demand, in some cases by up to 70% in urban contexts when compared with observed outcomes. Despite this, the quasi-scientific application use of trip rate databases, means trip rates are often presented as “robust” (or worst-case) and this practice continues to underpin the assessment process.

The issue is not the use of data itself, but how it is applied. Some popular trip-rate databases have become embedded as a default tool, often used in a way that implies a level of precision and certainty that may not be justified. The result is a tendency toward conservative forecasting, which in turn can drive the design and mitigation of schemes around theoretical levels of vehicular demand that do not materialise in reality.

This has wider implications. If assessments consistently overstate traffic impacts, there is a risk of over-provision of highway infrastructure, misallocation of investment, and missed opportunities to prioritise more sustainable modes. It also raises a more fundamental question: are current approaches inadvertently constraining the very outcomes that vision-led planning is seeking to achieve?

The work being led by the Transport Planning Society’s Development and Land Use Planning (DLUP) Group is directly engaging with these issues. Through ongoing dialogue with the Department for Transport and MHCLG, the group is exploring how transport assessment practice needs to evolve in response to a vision-led framework. A central focus of this work is understanding whether the incumbent approach, particularly around trip generation and vehicular impact assessment, has contributed to systemic bias in forecasting and infrastructure provision.

We are therefore calling on practitioners, researchers, and local authorities to contribute evidence and insights that can help test these assumptions. Do you have data available to show how accurate forecast trip rates are in comparison to observed outcomes, perhaps from travel plan surveys, ATCs, or other monitoring data? Are there consistent patterns of overestimation, and how do these vary by location, land use, or policy context? What examples exist where developments have demonstrably achieved lower vehicular trip rates, particularly where sustainable transport has been integrated from the outset?

The TPS-DLUP LinkedIn group is providing a forum for this discussion, providing a space to share case studies, data, and practical experience from across the profession. Contributions do not need to be formal; project-level insights and post-implementation observations are equally valuable. Please – engage in the discussion and help provide practitioner insight to shape effective policy.