Traffic Engineering Correspondence

Default values were used to prepare the SIDRA model provided with the traffic report. This includes the default
critical gap acceptance and follow up headway of 7 seconds and 4 seconds respectively for vehicles turning
right from the minor road (Holts Road). With two-way sign control calibration factors the output values are
6.24 seconds and 3.55 seconds.

Austroads Guide to Road Design Part 4A: Unsignalised and Signalised Intersections identifies these values for
a two way two lane road as 5 seconds and 3 seconds respectively. Table 5.10.5 in the Sidra Intersection 6.1
User Guide identifies the default values for a two lane – two way road is 5.5 seconds and 3.5 seconds. These
values are lower than the output critical gap acceptance and follow up headway with default values. Using
alternative values is therefore warranted. Applying a critical gap acceptance and follow up headway of 6
seconds and 3.5 seconds is considered reasonable.

4. Alternative Traffic Scenarios

Of the scenarios assessed with TTM’s traffic report, only the 2029 PM Development Case has been found to
result in unacceptable performance parameters. Once the -5% grade on Holts Road approaching Myall Street
is set to 0%, all other cases assessed in the traffic report still operate suitably. As such, the 2029 PM
Development Case will be analysed further. The following alternative model inputs, focussing on different
aspects of the SIDRA model, will assessed:

1.   Alternative growth rates on Myall Street at Holts Road and applying a 15% reduction for shared trips;
2.   Applying an 18% reduction in through trips on Myall Street at Holts Road as noted in Table 1;
3.   Design 2014 peak hour through volumes for the ‘upgrade option’;
4.   Using a gap acceptance and follow up headway of 6 seconds and 3.5 seconds respectively; and
5.   All alternative values in Table 1 used.

Further information on these scenarios is provided below.

Scenario 1: Alternative Growth Rates on Myall Street and 15% Reduction for Shared Trips

In this scenario TTM applied 1.7% and 2.8% p.a. growth rates for westbound and eastbound traffic
(respectively) on Myall Street at Holts Road. A 15% reduction was also be applied to traffic generated by the
service station and fast food establishment to account for customers who access both uses. Whilst applying
these alternative values, the following input data is still considered to be conservative:

        Heavy vehicle percentage;
        Base traffic volumes;
        Traffic generation of industrial land use;
        Fast food traffic generation.