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Pass-by and diverted traffic, and secondary trips | |||
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Pass-by, diverted and linked trips - Summary Not all the traffic that is predicted to use a proposed development will be 'new' to the locality, as some would be passing by anyway, or making a short diversion. Also, predictions of traffic to a mixed development could mean double counting as some people might use two land uses in the development on one trip. But it's a very inexact science and this page attempts to give guidance as to what might be realistic. | ||
Terminology - different types of trip The traffic prediction aim of a Transport Assessment (TA) is to assess how much extra traffic would appear on the local road network as a result of it being built. Most TAs correctly argue that not all traffic using the proposed development would be new to the road network.
The diagram below shows different trip types, using phraseology that seems to be most commonly accepted (though the numbering or diverted trips is mine). | |||
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Traffic to some developments will be largely due to people who are passing by anyway – a petrol station is an obvious example. Most new developments do not generate entirely new journeys but change the day to day travel patterns that people are making already. But unless a development is
then it is inevitable that it will draw traffic in from a wider area, if it to succeed. The question is - "how much?" The number of trips that are calculated as not adding traffic to local roads can have a very large impact on calculations of how much extra congestion will ensue. Hence the scope for argument and disagreement between TA writers and those who doubt the accuracy of a TA is very large too. Why are these trip types important? When a development is likely to attract a lot of traffic to it, the effect on the local traffic network will be less if many of the trips are pass-by or diverted. The diagram below shows how different trip types are counted as adding traffic to a local network, or not, as the case may be. | |||
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So in terms of additional traffic associated with a proposed development:
So if consultants can show that a high proportion of trips are likely to be pass-by or diverted, then the effect on junctions in the area will be less than if all trips were new to the local road network. The problem is that estimation of how many trips will be of different types is very difficult to assess in many cases, and research has been patchy. Some surveys have been done asking people whether they were passing by a development when they visited it, or whether they made a diversion, but;
Pass by, Diverted and Linked traffic literature Some Transport Assessments quote research studies in ways that do not give an accurate picture of likely rates for a proposed development. Common mistakes are to quote high figures, but ignore key factors that relate to the propensity to make linked, pass-by or diverted trips. Important factors relate to:
Key points from oft quoted key literature Below is key literature that is often quoted, looking at the characteristics of the developments and areas in which they are based. The table below summarises key literature, while paragraphs below provide more detail for many of them. The comments tend to critical, not of the research itself, but how the figures are used in TAs. |
Document |
Key finding quoted |
Comments |
Ghezawi,RS, Wegmann FJ and Chatterjee A (1998) Convenience Store Trip Generation, Institution of Transport Engineers Journal, USA |
Average pass by rate of 72% |
Based on American convenience stores with fuel station attached with average floorspace of 200m2. Very few UK developments would have anywhere near this pass-by rate, but it is very often quoted. |
Mouchel (2009) Proposed Tesco Store and Shopping Centre, West Bromwich, Working Paper 3 – Linked Trips |
40% robust estimate for PM peak |
This was part of a TA, arguing a level for a particular development, and may or may not be a robust estimate for that one, but that does not mean it will be robust for any other. Like many reports it is loosely based on findings from other reports. |
Trics and Safeway report 95/3 Traffic and parking at food retailing |
Usually one or two at the higher end of the spectrum for linked trips |
The study was of several stores, each of which had different relationships to their local centre, and very different linked trip rates |
Wrigley, Prof Neil, Dr. Dionysia Lambiri and Katherine Cudworth (2010) Revisiting the Impact of Large Foodstores on Market Towns and District Centres, University of Southampton (Available as Full Report, Summary Slides, December 2010, and Executive Summary) |
Various rates of linked trips reported, usually from the summary report |
The summary reports only touch the surface of the complexity of the findings, which are very different in different case studies. More is written about this and other work by Neil Wrigley in the text after this table. In particular they argue that linked trips boost town centre performance, which implies linked trips are a result of synergy, and not necessarily reducing overal trip making. |
Wrigley N, Lambiri, D and Cudworth K (2009) Linked trips and town centre viability. Town and County Planning 2009, pp 433-438 |
Not often quoted |
Quite in depth detail on Shepton Mallett case study. Shows figures quoted are for all modes, with walk linked trip rate much higher. Walk rate was 47% but car rate was only 27% |
Wrigley N, Cudworth K and Lambiri, D (2010) Further evidence on linked trips and foodstore development. Town and County Planning 2010, pp187-193 |
Not often quoted |
This gives more detail on the Somerset market towns case studies than is in the summary report |
MacIver A (1999a) Transportation Impact Assessment and trip-chaining behaviour, Traffic Engineering and Control March 1999 pp129-134 |
Seldom quoted |
This offers a re-analysis of data from a Sainsburys study of several supermarkets in Swindon and shows that around half of 'pass-by and diverted' trips were actually an extension of another trip, so should be counted as a primary trip |
MacIver A (1999b) Transportation Impact Analysis: Forecasting Travel Demand, TEC May 1999, pp 262-266 |
Not often quoted or quoted disparigingly as not being based on evidence |
This suggests likely pass-by rates for supermarkets in different locations, based on work in the preceding paper. TAs sometimes state that the figures are not based on evidence, though this is no less true than many other reports quoted |
TRICS Consortium (1995) TRICS Research Report 95/2 “Pass-By and Diverted Trips: A Resume” |
Quoted selectively as a source of figures, but also as now being out of date |
This was the best general review of literature and evidence on pass-by and diverted rates, and being produced by Trics, was the nearest to 'guidance' that existed, until the next entry below ... |
TRICS Consortium (2014) TRICS Research Report 14/1 “Pass-By & Diverted Trips” |
Not yet worked its way through to many TAs |
The report looks at literature in a somewhat uncritical manner, includes some new research, but concludes that each case has to be looked at separately with some guidance as to what factors might affect rates |
Guy (2007) Planning for retail development – a critical view of the British experience. Routledge, Oxford |
Only generally quoted as underplaying extent of linked trip making |
Tables provide summaries of other research with levels of linkage of trips for car users and non car users, and the types of trip for which linking is most commonly made (see below) |
The next paragraphs provide some more detail on literature referred to above.
Ghezawi,RS, Wegmann FJ and Chatterjee A (1998) Convenience Store Trip Generation, Institution of Transport Engineers Journal, USA
Johnson, Kevin L; Hammond, Matthew I (2001) Trip-generation characteristics for convenience stores, Institution of Transport Engineers Journal, USA
Mouchel (2009) Proposed Tesco Store and Shopping Centre, West Bromwich, Working Paper 3 – Linked Trips
Trics and Safeway report 95/3 Traffic and parking at food retailing
Wrigley, Prof Neil, Dr. Dionysia Lambiri and Katherine Cudworth (2010) Revisiting the Impact of Large Foodstores on Market Towns and District Centres, University of Southampton (Available as Full Report, Summary Slides, December 2010, and Executive Summary)
Wrigley et al found high levels of people who use the new supermarkets also sometimes using other services in the town on the same trip. But it should be pointed out that:-
Using a formula whereby “always or frequently” is assumed to provide a linked trip in 6 out of 10 trips and “occasionally” for 2 in 10 trips, the data suggests that for all trips the market town linked trip rate would be about 35%, and for car trips about 29%. For the District centres the figures would be 20% for all and 17% for car users.
MacIver A (1999a) Transportation Impact Assessment and trip-chaining behaviour, Traffic Engineering and Control March 1999 pp129-134
These figures are often criticised in TAs as not being based on evidence, though they are scaled by the findings of MacIver’s research from the Swindon Sainsbury’s study, and based on considered judgement.
TRICS Consortium (1995) TRICS Research Report 95/2 “Pass-By and Diverted Trips: A Resume”
TRICS Consortium (2014) TRICS Research Report 14/1 “Pass-By & Diverted Trips”
Guy, Cliff (2007) Planning for retail development – a critical view of the British experience. Routledge, Oxford
National Retail Planning Forum (2004) Linked trips and the viability of centres of retail activity. (Written by Oxford Institute of Retail Management)
Comments arising from literature Two things are noteworthy from the literature:-
Also some litarature points to findings which may give implications that are not as straightforward as they initially seem. The Wrigley analysis is taken by Transport Assessment writers to imply that developments close together will reduce trip making levels, but Wrigley argues that the juxtaposition of retail activity boosts the vitality of the area. Thus, while some might argue that linked trip making may reduce trips by, say, 40% of what each would generate, the synergy may actually lead to people being attracted to both facilities, but using the other as well. Thus, the total trips may only be slightly less than the sum of the two individual ones. Indeed some argue that development of one land use, eg a cinema, is important in attracting people to another, eg restaurants. ... So - therefore MORE trips. Summary of literature findings The only real conclusion that can be drawn is that there is not enough evidence to provide any accurate estimates of pass by, diverted and linked trip rates or percentages for developments. However I feel that it is fair to say that any rates over about 25-30% for pass by and diverted and similar levels for linked trip making should be treated with scepticism.
Any TA providing figures of this level or higher should be accompanied by a strongly argued case. And referring to precedent of other cases where such levels were not questioned by Local Authorities does not imply that the figures are realistic. It is more likely that the Local Authority were understaffed and/ or bamboozled by the TA.
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Analysis of data on pass-by, diverted and linked trips A section will hopefully be placed here at some point describing special analysis carried out on the GB National Travel Survey, showing the percentage of different types of shopping trip that may be pass-by or diverted trips. |
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The next page is concerned with some aspects of TAs
that may be of interest or use
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Gordon Stokes, 2015 |