PTOLEMY Pennbury
11642038
Validation of the PTOLEMY network traffic model around
Pennbury
Project note:
009
Technical note:
05
P:\P20\2038 PTOLEMY Pennbury\RECORD\TechNote\TN05_009_Pennbury Validation
Note_v2.doc
Version
1.0
Date
03/07/08
By
DF
Reviewed
Authorised
by
(TN only)
Ying Jin
Ying Jin
1.0_AS
Notes
Andy
Skinner
2
1.1
8/7/08
Dunc
Ying Jin
Ying Jin
Added revised validate flow plots from run 2006V3
INTRODUCTION
The existing PTOLEMY Network Traffic Model validation has been focused upon the cordons and
screenlines in the three City areas. For the Pennbury project, further road traffic validation has been
undertaken prior to the running of the housing tests, as the local road traffic counts in the vicinity of
Pennbury were not available at the time of original PTOLEMY validation. This helps to ensure that
the model results are fit for purpose for the Pennbury tests.
Road traffic validation consists of checking of the modelled road traffic loadings under congested
traffic conditions. To carry out this validation, the PTOLEMY model is run from its calibration year
(2001) to its Base Year (2006) in forecasting mode. The validation has been carried out primarily
using local traffic survey data supplied by the Leicestershire County Council.
Given that the model is run in forecasting mode (i.e. in an identical way to running the model to
produce future year forecasts), none of the surveyed data used in model validation has been used in
calibrating the model. Also, the travel demand matrices used for car traffic assignment are
synthesised matrices that have been forecast by the PTOLEMY travel demand submodel using land
use activity and regional economic inputs. No matrix-estimation techniques (such as used within
SATURN modelling) have been applied to alter the synthesised demand matrices prior to traffic
assignment.
TRAFFIC COUNTS: VALIDATION FOR CARS AND ROAD GOODS VEHICLES
The surveyed traffic count data used for the Pennbury road traffic validation has been obtained from
the following sources:

Local surveys of road traffic by hour for some road links in the Pennbury area (2004-2008)

This is supplemented by the DfT road traffic database, which records manually classified road
traffic counts by hour for 12 hours a day on the basis of a rotating census (2000 to 2004) on a
limited number of links
At some sites there is data from both the local survey and from the DfT’s road traffic database . In
such cases local survey data (which is more recent and generally considered more reliable) is
always used in preference to the earlier DfT data.
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The total number of trips modelled was validated in terms of car volume and total traffic in equivalent
private car unit (PCU). Powered two-wheelers (motorcycles and mopeds) are included with the cars,
in accordance with the definitions used for PTOLEMY modelling.
The modelled traffic flow data also includes exogenous load – which is estimated based on road
counts to include the effect of goods vehicle and car traffic which are not included in the modelled,
endogenous demand matrices 1. The exogenous loads are calibrated along each motorway and
trunk road corridor on the basis of the difference between the observed traffic counts and the
assigned, endogenous road traffic.
All PTOLEMY model results in this section are from the model outputs for the Base Year of 2006
(Version 1-02-03. internal reference /v123/2006/AM_v03)
Key to modelling transport impacts of housing developments within Pennbury is the assurance that
traffic flows on the A6 and A47 match well with the counts. Results are presented for cars and total
vehicles in terms of the % difference between the modelled traffic counts and the observed traffic
counts and the GEH statistic [see Design Manual for Roads and Bridges, Vol12, Section2, Part1]

Figure 1 shows the percentage difference for car traffic

Figure 2 shows the GEH statistic for car traffic

Figure 3 shows the percentage difference for all vehicles (measures in pcu)

Figure 4 shows the GEH statistic for all vehicles (measured in pcu)
1
This additional traffic could arise from i) traffic that is travelling right across the PTOLEMY area on major
roads, ii) intra-zonal traffic within large zones which is not assigned endogenously onto the network and iii) a
small amount of road goods vehicle traffic that has not been included in the good vehicle matrices; this
primarily applies to areas outside the 3 Cities. Item i) implies considerable traffic volume on the trunk roads,
whilst ii) and iii) are relatively minor.
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Figure 1: Comparison between the observed traffic counts (total car) and the modelled flows
in the morning peak. The percentage difference is shown
Figure 2 Comparison between the observed traffic counts (total car) and the modelled flows
in the morning peak. The GEH statistic is shown
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Figure 3: Comparison between the observed traffic counts (pcu) and the modelled flows in
the morning peak. The percentage difference is shown
Figure 4: Comparison between the observed traffic counts (pcu) and the modelled flows in
the morning peak. The GEH statistic is shown
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