BOARD of INQUIRY BASIN BRIDGE PROPOSAL Attachments To
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BOARD OF INQUIRY BASIN BRIDGE PROPOSAL Attachments to evidence of John Foster for Save the Basin Campaign Incorporated Index Item Page Traffic Operations in Bottle Necks 001 Transport Models and Intersection Analysis 018 Page 1 Attachment A: Traffic Operations in Bottle Necks Overview: 1. This technical note provides background data and evidence to support my contention that both the SATURN and S-Paramics models are seriously flawed in that they fail to replicate critically important traffic conditions in the vicinity of the proposal. 2. I have assembled from past studies and direct observations a detailed description of the bottleneck conditions on the approaches to Wellington City and particularly that affect approaches to the Te Aro area. Downstream of the bottlenecks peak hour flows cannot change. This creates a relatively calm and settled traffic environment in the Te Aro area including the Basin Reserve. This will persist unless or until the current RONS programme is delivered and especially until the proposed duplications of the Terrace and Mount Victoria tunnels are completed. Following duplication of the tunnels, pent up upstream demand will be released and in my opinion will have a significant impact on traffic in Wellington City and especially in the Te Aro area. Modelling Bottlenecks 3. In my view, modellers for the Proposal have not properly addressed the effect of bottleneck conditions on traffic operations. The effective performance of the Wellington Regional arterial network depends primarily on the ability of the major intersections to efficiently cope with everyday traffic demands. Currently many of these critical intersections are overloaded during much of the morning peak periods on weekdays. Bottleneck conditions have been prevalent at Pukerua Bay, Mana Esplanade, Ngauranga Gorge, Petone On Ramp, and Mount Victoria Tunnel for at least the past 10 years. Only those at Mana have been corrected by widening the affected roadways to increase their capacity. 4. In nearly all cases a merge manoeuvre is involved whereby the joining of two of more traffic streams exceeds the capacity of the receiving roadway for a short period. I have personally measured the details of the bottlenecks listed 001 Page 2 above. The following table shows the observed capacity limit, the period over which this capacity is exceeded, and my rough estimate of the peak demand of the incident flows. I have not included the bottlenecks that occur in the Terrace and Mount Victoria tunnels, as these are described in detail later in my evidence. Nor have I included many small period bottleneck situations where capacity limits apply for periods significantly less than 60 minutes. 5. The estimates of likely peak demands have included three components, the excess demand currently stored upstream of the bottleneck, an allowance for the return of traffic diverted to otherwise less desired routes, and an assessed increase due to induced traffic effects. It is unlikely that such latter increases would exceed 5%, but the very substantial reduction in travel times that would accrue should the flow restrictions at Petone and within the Ngauranga Gorge be removed could engender a substantial shift from public transport. PEAK/ CAPACITY PERIOD PEAK DEMAND LOCATION (veh/hr) DIRECTION (veh/hr) (mins) AM 1,600 50-70 1,700-1,850 SH 1 - Pukerua Bay northbound AM 1,480 80-100 1,950-2,300 SH 1 - Mana Esplanade southbound PM 1,480 100-150 1,900-2,100 SH 1 - Mana Esplanade northbound AM 6,000 95-110 6,500-8,500 SH 1 - Ngauranga Gorge southbound AM 4,000 100-120 5,000-6,000 SH 2 - Petone On Ramp southbound 6. The following section contains a detailed technical description of the real world traffic flow conditions that apply northbound during the morning peak period over the length of State Highway 1 stretching from about midway along Cobham Drive to the Basin Reserve. It is necessary to firmly establish the actual nature of every day traffic events to provide an objective benchmark for the detailed review of the virtual world of the traffic models discussed in later sections of my evidence. 002 Page 3 Characteristics of the Traffic Stream 7. It can be shown with mathematical rigour that the flow of traffic along a uniform roadway is completely described by only three variables flow rate, speed, and density. It is convenient to express flow rate in vehicles per hour, speed as the average speed in kilometres per hour, and density in units of vehicles per kilometre. It can be further shown that these three variables flow rate (q), speed (u), and density (k) are related by the equation - q=u.k . 8. Research by scientists at General Motors Corporation in the late 1950’s showed that if the traffic stream is constrained by disallowing overtaking on a crowded road, that cars follow according to a simple rule. The following car accelerates or slows according to the rate at which the car in front appears to be getting farther or closer to the driver of the following car. The response is then directly proportional to the relative speed between the vehicles and is delayed by the reaction time of the following car-driver combination1. 9. The car following law implies that there exists a direct relationship between two of the fundamental variables in that speed is inversely proportional to the density. In mathematical terms speed equals a constant times the density and the flow rate is proportional to the density squared; or is parabolic in form. 10. The likely relationship of flow rate versus density for a single lane when confined, say in a tunnel; is illustrated in Figure 1 below. The dashed portion identifies that portion of the relation where the necessary conditions of crowding and no overtaking are not met. In these circumstances spacing and headways are not related but are independently random. The data points have been obtained from the TMS site in Ruahine Street described in paragraph 13 below. 1 A reference will be provided to this analysis. 003 Page 4 1800 1600 1400 ) 1200 1000 800 Flow Rate (Veh/Hr Rate Flow 600 400 200 0 0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0 Density (Veh/Km) Figure 1: Flow Rate - Density Relationship 11. The function has a maximum value, termed the saturation flow rate, or simply the capacity. In this case, a flow rate of 1600 vehicles per hour (veh/hr) at a density of 45 vehicles per kilometre (veh/km) with a speed of 35 km/hr. In addition, the relation displays two crucially important properties: a) For any value of the flow rate less than the capacity there exist two valid states of differing density. For example a flow rate of 1100 veh/hr can exist at a density of 20 veh/km with a speed of 55 km/hr, or at a density of 70 veh/km with a speed of 16 km/hr; b) The densities along the right hand arm are significantly greater than those along the left hand arm of the relation. 12. Real world drivers are conscious of the relative proximity of other vehicles in their lane. Density rather than speed or flow rate has been adopted by the USA Highway Capacity Manual as the determining measure affecting the Level of Service (LOS) of high level facility roadway types Drivers perceive densities 004 Page 5 greater than that of the maximum flow rate as “congestion”, whereas in strict traffic engineering terms congestion occurs only when demands are near or exceed the saturation flow rate. The Mount Victoria Tunnel Bottleneck 13. Periodically the NZTA traffic monitoring sites are set to record individual vehicle speeds in 15 minute time slices. Observed values are stored in one of 17 speed bins of <30kph, 15 bins at 5kph intervals from 35 to 95 kph, and one when speeds are >95kph. The average speed of the traffic stream can then be monitored throughout an entire day. Figure 2(a) below illustrates such information for northbound flows during the morning peak period on Thursday 17th of August 2012 at the TMS site on Ruahine Street located south of Goa Street. Traffic operations were similar to those described in Figure 2(a) for all the other weekdays of that week. 14. Figure 2(a) shows the counts as a bar graph converted to equivalent hourly flow rates by factoring the 15 minute tallies by 4. The average speed is plotted as the red line referred to the alternate vertical axis located on the right. The density plotted as the solid blue line has been calculated according to the basic formulae by dividing the flow rate by the average speed. The Figure thus depicts the dynamic variations in the three basic flow variables over the period extending from 6:30 am to 9:15 am. 005 Page 6 1600 80 1400 70 1200 60 1000 50 800 40 600 30 400 20 Equivalent Flow Rate (vph) Rate Flow Equivalent 200 10 (vpk) Ddensiy and Speed(kph) 0 0 6:30 6:45 7:00 7:15 7:30 7:45 8:00 8:15 8:30 8:45 9:00 9:15 Volume (vph) Speed (kph) Density (vpk) Figure 2(a): SH 1 –Northbound (towards CBD) AM Period - Ruahine Street 15. The speed reduces steadily from 60 kph at 6:30 as the flow rate increases then falls suddenly from about 45 kph at 7:30 am to near 20 kph, and remains at this low value until about 9:00 am when it suddenly increases back to near 50 kph. As a direct consequence the density suddenly increases, stays nearly constant for the 90 minutes between 7:30 am and 9:00 am, then falls back to a low normal value.