When a site already has a use, the question for highways is not how much traffic the new scheme creates, but how much more (or less) than what is there now, or what could lawfully be there. Getting this right can turn an objection into no objection.
The existing use
If a site has a lawful use, such as a workshop, a shop or offices, the traffic it can generate is part of the baseline. A redevelopment that replaces it may add very few trips, or even reduce them. National trip rate data for the existing use, or surveys of it while it is still operating, provide the evidence.
The fallback
A fallback is what could happen on the site without the planning permission being sought, for example reopening a vacant use, or development allowed under permitted development rights. The courts have confirmed that a fallback can be a material consideration where there is a real prospect of it happening. The leading Court of Appeal judgment is Mansell v Tonbridge and Malling Borough Council [2017]. A theoretical possibility is not enough.
Where it makes the biggest difference
- Redeveloping commercial or industrial sites for housing
- Changes of use between business, retail and leisure
- Farm buildings with a working agricultural use
- Sites with an extant planning permission that could still be built
How to make the case
- Establish the lawful use, with planning history or a certificate of lawfulness if needed.
- Show the traffic it generates or could generate, using surveys or recognised trip rate data.
- For a fallback, show it is a real prospect: viable, likely and allowed.
- Compare like with like: same hours, same vehicle types.
What officers look for
Officers are rightly cautious about inflated baselines. A long-vacant use or an unrealistic fallback will be challenged. A fair, evidenced comparison is far more persuasive than the biggest possible number.
General information, not advice on a specific site. Policy and guidance correct as of October 2026.