How Highways assess the traffic impact of a new development

The Parish Council contacted the Highways department at Lincolnshire County Council to ask how it assessed the impact that a proposed new housing development would have on the traffic in Woodhall Spa and we received the response below:

When the Transport Assessment is submitted as part of a planning application that TA has to take into account all other permitted development within Horncastle over a certain size i.e. all other sites which required a TA.  This is so we get an idea of the cumulative impact of all developments which have planning permission regardless of whether or not they have been built out.  So the Witham Road application would have had to take in to account the traffic associated with all the other approved developments in Woodhall Spa in addition to their own predicted traffic.

We also compare these recent TA’s against the one submitted by the applicant to ensure that the traffic counts contained within the TA are comparable with other TA’s submitted which cover the same junctions.  These counts have to be taken on a neutral day to comply with the guidance given to compile a TA.  A neutral day is an average working day which excludes school holidays, bank holidays.  There are in fact only around 89 neutral days within the calendar year.  We all except and appreciate that the average working day traffic does not compare with the traffic experienced on a busy Bank Holiday, however, developments should not be refused on the basis of traffic impacts based on around 12 days of the year, hence why the governments advice is to use a neutral day.

The impacts of the traffic at identified junctions which are agreed in advance of the TA being undertaken are assessed and the maximum peek 15 minutes are identified  whether this is in the AM peek or PM peek and these figure are then used to assess the capacity of that junction.  The junction is deemed to be operating efficiently and within capacity whilst the RFC (Ratio to Flow to Capacity ) is below 0.85  (85%) of its theoretical capacity.  Once it hits this mark we will ask the developer to look at mitigation measures to see if or what can be done to improve the junctions capacity.  When that figure hits 1 then the junction is considered to be operating at capacity and then it has to be assessed to see if the impact on this junction is serve in line with the guidance contained within NPPF or if there is what is known as peek spread occurring which means the junction will be operating at capacity for longer than normal and what effect this has on the surrounding highway network.

I would like to point out that with the governments drive to provide new housing we are under pressure to allow new developments unless the impact is “SEVERE”.  if there are 20 vehicles queuing to get through a junction and you add another 10 is this severe?  Recent appeal decisions would suggest not.

I appreciate that in East Lindsey we are generally blessed with low traffic flows and as such when we do get a new development we are concerned about the traffic impacts, however, when taken in context with the rest of the country we are still relatively lightly trafficked.

The following document explains what Highways do and what it considers when looking at the potential impact on highways from any new proposed development

how-highways-assess-a-junction