FUNCIONAMIENTO DEL QUEMADOR PUESTA EN MARCHA DEL QUEMADOR (A)

The UseMethodImpAtNode function that calculates the impedance at a node was kept unchecked during the trip assignment procedure for very specific reasons. During the assignment with ICA in oversaturated networks, the model limits the inflow on the links; it does not allow the volume passing from one link to another to exceed the capacity of the link, nor does it allow the queues on a link to exceed the stocking capacity of the link. These constraints unnecessarily complicate demand matrix correction when observed volume counts are available, as was the case in this research project. This means that at all the nodes and turns, tcur equalled t0 (set to equal zero), and only the impedances on

the links were thus considered during the iterations of equilibrium assignment. Only after the trip- assignment and demand-matrix-correction procedures were done, was an ICA calculation executed (one at a time) at each node and main node that is controlled either by signalisation, a roundabout or two-way stop. The analysis was performed over a one-hour analysis period that coincided with the AM peak hour that was previously defined. This approach implicitly assumed the arrival rate of vehicles within the hour to be constant throughout. Time did not allow for dividing the one-hour analysis period into four 15-min analysis periods and then determining the control delay at every node for each of the periods and for every scenario described in Section 4.5.2.2. The PHFs at all of the intersections suspected to cause the greatest delays ranged from 0.88 to 0.97, with most of the PHFs falling into the upper limit of this range. These high values were taken as an indication that the one-hour-analysis- period approach was acceptable to use.

The standard formats of the ICA reports for signalised intersections, roundabouts and two-way stops, respectively, are presented in Appendix D.4 as Tables D.1, D.2 and D.3, along with the formula for each of the input parameters. The LOS criteria for these three control types are given in Tables 4.6 and 4.7. The methodologies for calculating control delays for the three control types, respectively, are taken directly from chapters 18, 19 and 21 of the HCM 2010, and are thus not repeated in detail in this write up. Although the calculations were set to be done according to the HCM 2010, some of the layouts of the steps follow the HCM 2000 more closely. An arrival type 3 was assumed for all of the signalised intersections: “random arrivals in which the main platoon contains less than 40 per cent of

Page | 95 Table 4.6: LOS criteria for signalised intersections.

Control delay (s/veh) LOS by volume-to-capacity ratio

≤ 1.0 > 1.0 ≤ 10 A F > 10 – 20 B F > 20 – 35 C F > 35 – 55 D F > 55 – 80 E F > 80 F F

Table 4.7: LOS criteria for roundabouts and two-way stop intersections.

Control delay (s/veh) LOS by volume-to-capacity ratio

≤ 1.0 > 1.0 ≤ 10 A F > 10 – 15 B F > 15 – 25 C F > 25 – 35 D F > 35 – 50 E F > 50 F F

the lane group volume”. Since the ICA reports only contain values (no formulas), and some minor errors

were found in a few of the reports, formulas were entered into the ICA reports for the base case. These reports were then copied for all the other scenarios, so that only changes to the base volumes (plus some other minor changes where necessary) had to be made in order to obtain the new delays. For all control types, the impact pedestrian movements had on the control delay was ignored, because it was rarely observed that traffic was delayed as a result of pedestrian movements alone. On the tertiary and residential roads, the pedestrians could simply walk through the stationary traffic. At the signalised intersections, very few pedestrian calls were made with the actuated push button. The only pedestrian crossing that was not ignored was the one at Paul Roos. The frequency of the pedestrian calls was recorded along with the signal timings. A pattern was observed for the majority of the hour and therefore, a pre-timed signal plan could be derived. This is the only ‘intersection’ for which the ICA calculation was eventually set to be performed during the assignment procedure, as it helped profusely in more accurately assigning vehicle trips to all the side streets in Krigeville. Also, at the Van Reede / Doornbosch intersection, it is near to impossible to get out of Doornbosch and a traffic officer assist traffic flow there every morning. The ‘phases’ were timed and an approximate average delay was applied for each of the turn movements.

Once the control delay for every movement at every intersection had been calculated, the values were entered back into the model as tcur for all the permitted turns. This was repeated for every scenario.

4.4.3.6 PERFORMANCE MEASUREMENT OF THE NETWORK

The future objective of the model was to measure the performance of the network for the project alternatives described in Section 4.5. In Visum, performance measurement can be divided into one of two types:

Page | 96 2. performance measurement for the entire network.

The former is calculated with skim matrices and the latter using global indicators. User-developed performance measures can of course also be done by examining the attributes of individual network objects and, for example, adding them to study the performance of a specific route.

In this research project, the three most important model outputs were:

1. Vehicle Miles Travelled (VMT) in the AM peak hour. VMT is calculated from the total number of vehicles in the network and the distance (in km) that they travelled.

2. Vehicle Hours of Travel (VHT) during the AM peak hour. VHT is computed from the product of a link’s volume and its travel time, summed over all of the links.

3. Mean system speed for the AM peak hour. The mean system speed is the quotient of VMT and VHT.

These outputs, more specifically VHT and mean system speed, served as the inputs to the equations that quantified the traffic conditions in monetary terms, i.e. the value-of-time and vehicle-operating- cost equations, which in turn were needed for the CBA. VMT, VHT and mean system speed are all global indicators. VMT and VHT were automatically calculated during the execution of the Assignment

Analysis procedure. The procedure had to be executed two times: once for the links and once for the

turns. The results for each were then summed to get the total VMT and total VHT. The same results could have been obtained with the execution of the Calculate PrT skim matrix procedure, and then adding the results of all the O-D pairs.

As mentioned in Section 4.4.1.2, probe data was applied to evaluate the average travel speeds and travel times of the three main alternative routes from the R44 / Van Reede intersection to the Dorp / Piet Retief intersection. The Calculate PrT skim matrix procedure was used to determine the same two attributes for the same three routes for the base case, so that a comparison between the probe data results and the Visum model results could be made for this case. Since the modelled results were used for all the other cases, this verification of the base-case results was very important. The paths of the O-D pairs do not directly match those of the three routes. The travel time on some specific links thus had to be subtracted or added from / to the travel time for an O-D pair.

Given that the mobility benefits had to be determined not only for the entire network, but also for the main routes that some commuters will continue to use in all of the alternatives, the individual travel speeds and travel times had to be evaluated for these routes for all of the scenarios. The routes comprised the probe data routes, but also the combination of paths of the 1-6, 1-7, 1-9, 1-10, 1-12 and 1-13 O-D pairs. The calculations were done in the same way as described above for the probe-data validation.

In addition, the performance of the major intersections was measured. Because the volume-to- capacity, control-delay and LOS calculations had already been done for all of the intersections, the measurement of performance merely comprised an analysis of these results. Although the traffic conditions had to be monetised for the CBA, an improvement or deterioration in traffic conditions is often made sense of better when expressed in units of speed and / or time. That is why the measurement of performance for the intersections was done only in terms of these units.

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4.5 PROJECT ALTERNATIVES

Three project alternatives were evaluated as part of this research project. Unquestionably, the theoretical implementation of a bicycle-sharing scheme for school and university destined commuter traffic in Stellenbosch was one of these. This alternative would not have been worth much without an alternative to compare to. This is why the null alternative – the do-nothing option was so important. The other project alternative was evaluated as a supplement, to put the results of the CBA for the bicycle-sharing scheme into further perspective. At a time when this research project was already well underway, the municipality began with the geometric improvement - capacity enhancement - to the R44 / Van Reede intersection. It thus seemed appropriate to include this geometric improvement as a project alternative.