5.3.1 Data used for the development of the S-Paramics model
It is often difficult to find sites at roadworks and with good vantage points to record data close to the taper section (for example, by filming from an overhead bridge close to the taper section). Therefore, for this research, historic data from two motorway roadworks sites (which were made available for this study) were used for the testing of the S-Paramics software.
Table 5.1 illustrates the details of the sites locations. The data were used in building and developing an S-Paramics model to demonstrate the capability of the software. The first site was on the M61 motorway northbound between Junctions 8 and 9. This site had a slight left hand bend just before approaching the roadwork section. Around three hours of video footage (from 15:20 to 18:30) on Friday, 16 August 1996 was recorded. The second site was the M6 at the southbound direction between Junctions 14 and 15. Four hours of video footage (from 8:00 a.m. to 12:00) was recorded on Friday, 25 October 1996. For both sites, a speed limit of 50 mph (80 km/hr) was imposed and an offside lane (i.e. lane 3) closure was applied, for further details on these two sites see Section 3.3.4. It should be noted here that although the data used for this study seem to go back a long time, it is believed that there have been no significant differences in the layout of roadworks closure to influence drivers’ merging behaviour close to the taper section, hence, the justification of using the historic data for the purpose of testing the S- Paramics software in this study.
Table 5.1: Roadworks sites details
Site No. Site locations Traffic direction Number of lanes Date Duration (hours) Speed limit Type of section 15 M61 (J8 - J9) Northbound 3 lanes 16/08/1996 3 50 mph Roadwork with offside lane closure 16 M6 (J14 - J15) Southbound 3 lanes 25/10/1996 4 50 mph Roadwork with offside lane closure
The average speed and traffic flow for each lane of the observed motorways have been extracted by playing back the two footages. Using a marker pen, a thin line across the computer screen was sketched in line with the point of start of the taper section (i.e. transition zone) at both roadwork sites to help in counting vehicles (e.g. measuring time headways between successive vehicles). Similarly, in order to calculate vehicles’ speeds, two screen lines were drawn to cover a distance close to 100 m. The first line was located at the start of taper, whereas the second line
was after 11 consecutive white road markings (9 m each) away from the first line. Simple distance/time calculations were then used to convert the readings into speeds using video playbacks with the help of computer programming codes written for this purpose to act as a stopwatch. The output was formatted and stored on a spreadsheet for further analysis. The data were then grouped into five minutes intervals (as suggested by previous researchers such as Yousif, 1993 and Al-Obaedi, 2012), and to ensure that the data were tailored to the needs of S- Paramics.
5.3.2 Building the S-Paramics model
Following data collection, the building of the S-Paramics model was achieved by setting out the geometry of the M61 site which is similar to the M6 one since both sites had the same traffic management scheme (i.e. offside lane closure) and with the same applied speed limit of 50 mph. To correctly model a section, an image from Google maps for the motorway site was used as a base to create an overlay within AutoCAD. This overlay was then inserted into S-Paramics to the correct scale. Nodes and links were then created. Links carry the geometric design and characteristics of traffic (e.g. number of lanes, directional movements, speeds … etc.), and nodes were created at points where the section changed (i.e. where one lane was withdrawn from the carriageway).
Two types of vehicles were modelled, cars and Heavy Goods Vehicles (HGVs), with their corresponding vehicular composition. The model was run for three hours for both sites (with the first half an hour used as warming-up and the last half an hour used as cooling-down periods). The option of having loop detectors within S-Paramics was used to gather output data for flow rate and average speed for each lane.
5.3.3 Modelling roadwork site layout
As mentioned in Section 2.3, most TTMSs are divided into four zones, namely, the advance warning zone, the transition zone, the activity zone, and the termination zone (MUTCD, 2009). Road closure (activity zone) is one of the options which seems to be directly available within S-Paramics. To model the effect of the presence of the taper section (transition zone) and the advanced warning zone, the option of the “signposting” within S-Paramics was used since there appears to be no other option in S-Paramics for modelling the taper section (Nassrullah and Yousif, 2015). This option (i.e. “signposting”) is used as a way of communicating to the approaching upstream vehicles in the simulation that there is lane closure ahead. It helps drivers
to reassess their lane choice and get in the correct lane before approaching the closure. According to SIAS Limited (2007), the signposting distance has a significant effect on drivers’ behaviour. The modelled drivers were made aware of the presence of the roadworks before they reached the start of the traffic management signage. This was done by increasing the signposting distance. However, according to Walker and Calvert (2015), this increase may not necessarily result in a change in drivers’ behaviour which implies that the use of signposting is not effective. Therefore, it is found necessary to have a closer look at the effect of the presence of a taper section and the manner by which lane changes are performed. This is shown in some detail within the next sections.
The default value of 750 m as the signposting distance was used. This value has been changed and tested using data from the M61 and the M6.