The Use of Differential Speed-Flow Curves in SATURN

The Use of Differential Speed-Flow Curves in SATURN KARL ILBREY 31 st October 2008

Introduction Work undertaken by the Highways Agency in 2005 showed the importance of using speed-flow curves, and the difference they could make to the results of an economic appraisal The developers of SATURN were asked to formulate a method for using differential speed-flow curves within the program Two different algorithms were developed and have now been assessed further

SATURN Algorithms CLICKS Where the light vehicles free flow speed is greater than a specified maximum limit the extra time taken is calculated and treated as a fixed delay for vehicles of that type CLIMAX The speed remains constant as long as it is lower than lights. When the light vehicle speed drops below the HGV limit, both travel at the same speed KLUNK The maximum speed is defined for each road index

Test Models A556 Mere Model – Knutsford to Bowdon Improvement Covers Cheshire and Manchester and includes several small towns Essentially in buffer apart from the M6 to M56 link under investigation and all motorway links / junctions Convergence just outside full DMRB criteria A14 CHUMMS Model – Ellington to Fen Ditton Improvement Covers Cambridge and Huntingdon with associated strategic routes Major route under investigation coded entirely in simulation Better convergence, though now superseded by a more upto date model M6/M62 Parkside Model – Junction 22 Amendment Covers urban areas around Newton-le-Willows and Warrington Includes M6, M62 and A580 East Lancashire Road Covers a smaller more urban based area

Test Methodology (1) SATURN Version Base model without differential curves Base model with CLICKS by vehicle type only SATURN Version Base model without differential curves Base model with CLICKS by vehicle type only Base model with CLICKS and KLUNK Base model with CLIMAX by vehicle type only Base model with CLIMAX and KLUNK

Test Methodology (2) The five different algorithms were run for: Do-Minimum and Do-Something situations And both… Opening Year and Design Year situations Tests on the Mere model were undertaken using both versions to enable a comparison of versions to be made For each assignment a note was made of the total Vehicle-hours and Vehicle-Km by vehicle type, plus flows and times along an A556 Improvement screenline The tests on the Parkside and CHUMMS models were both confined to options using KLUNK in version

Aspects under analysis… 1.Has the update from to made any changes? 2.How much do the different algorithms affect the assignment results? 3.Are the changes sufficient to have a significant effect on the economic appraisal results?

Comparison of SATURN Versions and Figures for Vehicle-Km and Vehicle Hours were compared for each User Class A comparison was made for SATURN both with and without CLICKS by Vehicle Type The change in version has made some slight differences to the assignment results However, the difference was less than 0.1% for all items measured, and less than 0.01% for most of them It could therefore only be significant in a model where the scheme is very small compared with the model size It was therefore concluded that the change in version had no effect

Effect of different Algorithms (1) This can be seen through the Percentage Changes in Vehicle-Km in the Base Year for the Mere and Parkside Models:

Effect of different Algorithms (2) It can also be seen through the Percentage Changes in Vehicle Hours:

Effect of different Algorithms (3) It would be expected that as only extra time is added by the algorithms, only the vehicle hours would change However, the extra time is clearly causing some route choice differences Even though the algorithms are only used on OGVs, Car and LGV times and distances are also being changed due to re-assignment As expected, the greatest changes are to the OGV Vehicle Hours, with upto a 12% increase caused by using CLICKS by Vehicle Type OGV vehicle distances can change by upto 1.8% CLICKS gives consistently bigger differences than CLIMAX, due to the smaller differential between the speed-flow curves The choice of algorithm can therefore have a significant effect on the results of an assignment

Effect on Economic Appraisals (1) The future year assignments were used to calculate the change in vehicle time and distance due to the relevant improvement scheme These then act as a proxy for the economic benefits, and indicate how much the SATURN parameters may affect TUBA results Assessments were undertaken for the A556 Improvement in both the opening and design years (2014 & 2029) The process was also undertaken using the Parkside and CHUMMS Models for just a single assessment year

Effect on Economic Appraisals (2) Changes in Design Year for A556 Mere Improvement (2029) There is little variation between the options assessed This is likely to be due to the level of congestion in the network, as the area is already subject to considerable delays

Effect on Economic Appraisals (3) Changes in Opening Year for A556 Mere Improvement (2014) There is much more variation between the options assessed in the opening year This is sufficient to imply that this could have a noticeable effect on economic results

Effect on Economic Appraisals (4) In order to confirm these results, the same analysis was undertaken for the Parkside Model

Effect on Economic Appraisals (5) The exercise was also repeated using the CHUMMS Model

Effect on Economic Appraisals (6) These two models give considerably less differential than the A556 Mere model The reasons for this are not clear, but it may be connected with the ratio of benefits to total model costs – for Mere benefits are only 0.1 to 0.8% of total model costs Thus, a small difference, perhaps caused by differences in convergence, can have a significant impact on benefits The other models show benefit to cost ratios of 0.5 to 2.7% and 3.5 to 12.7% so convergence differences will have less effect In some models the choice of algorithm may have a significant impact on the economic benefits of a scheme

Conclusions The change in SATURN version has had no effect The choice of algorithm can have a significant effect on the results of an assignment In some models the choice of algorithm may have a significant impact on the economic benefits It is therefore suggested that the Department for Transport may wish to recommend that a sensitivity test should be undertaken with both CLICKS and CLIMAX to check whether the choice will have affected economic benefits

Presentation Title here I date here The Use of Differential Speed-Flow Curves in SATURN KARL ILBREY Tel: +44 (0)