Políticas estatales con los pueblos indígenas

The existing simplified procedure combines relationships for passing supply and demand together to determine the Unsatisfied Passing Demand (UPD) in each road segment. This is then applied to the segment length to determine the relative increase

that APD cannot fall below zero). When comparing passing lane options, this therefore generally leads to parallel changes in APD following a passing lane, with the two options never meeting.

Figure 3.11 illustrates this scenario. Given that passing lane effects generally diminish downstream over a fixed length, this result is not producing a realistic model of the situation.

Figure 3.11 Typical derivation of APD (Accrued Passing Demand).

Running Distance (km)

Accrued Passing Demand (APD)

Two Options Remain Parallel Without Passing Lane With Passing Lane Initial APD Slope = UPD per km Demand > Supply

(APD accrues) Demand < Supply

(previous APD dissipated)

Another potential problem is that a road with poor existing passing opportunities and high traffic volumes can suffer from constantly high UPDs, with the net result that the APD continues to increase unconstrained. Notwithstanding the previous discussion on average passing demands per queued vehicle, there is clearly a limit to how much APD is present at any given point along the road, particularly when the proportion following is not observed to be exceptionally high. This is because only a finite number of vehicles are present.

Section 3.3.4 will consider the “passing supply” side of the equation, particularly with regard to maximum passing rates. Let us consider for now, how the “passing demand” is derived. The number of “catch-ups” determines demand. The Passing Demand, DA-B, (i.e. the frequency with which vehicles in stream A catch up to vehicles in stream B) is determined by:

DA-B = Z × KA× KB×σA (catch-ups/km/hr) (7) where:

Z = a constant, based on relative differences in speed between the two streams A, B KA = Traffic Density = {HourlyFlow}A / {MeanSpeed}A (veh/km) for streams A, B

Often the two traffic streams analysed are “cars” catching up to “trucks”. This formula can also be simplified to determine catch-ups within a traffic stream, e.g. cars catching up to slower cars.

The original basis for these calculations was for low traffic flow cases where single vehicles caught up to other single vehicles and were likely to be able to overtake relatively soon after catching up. A maximum one-way flow of 150 veh/hr, or a mean bunch size of less than two, was recommended for using this model in order for the results to be reasonably valid. Given that, in many cases, the flows modelled are likely to be somewhat greater, the true demand needs to be considered in these cases.

When determining catch-up rates when some vehicles remain bunched, remember that only the lead vehicles or free vehicles will be dictating vehicle interactions. The other vehicles already bunched are not affecting the additional catch-ups generated, and could be excluded from the passing demand calculations, if we concentrate only on the minimum passing demand and do not include within-queue interactions.

As the proportion of bunched vehicles increases, the proportion of vehicles generating catch-ups should decrease. For a given % following, f, there are

(1-f)×{Volume} vehicles free to interact. The calculated passing demand is hence

(1-f)×DA-B. If we assume that APD is proportional to bunching and volume, then this

calculation could be re-expressed as:

{TrueDemand}A-B = (1-APD/{OneWayVolume})×DA-B (8) The net effect of this approach is that, at relatively high levels of APD, the passing

demand generated is much lower than at low levels of APD for the same road segment. This has the effect of lowering the UPD and subsequently not increasing the APD by as much (or decreasing it by more). A passing lane option that will substantially lower the APD is likely to cause a quicker return downstream to a higher APD than that for the same road segment without the passing lane.

This results in the two options converging again at some point downstream, and Figure 3.12 illustrates this effect.

The above approach will also help to minimise the rate of climb in APD as, when APD is approaching the actual volume, UPD values will be scaled to near zero. Because of the non-continuous nature of simplified passing lane procedures, where road segments can often be a few kilometres in length, an additional cap will be necessary to prevent APD over-runs. Given that it is highly unlikely to see bunching greater than 90%, a reasonable approach is to cap APD to a maximum of 0.9×{OneWayVolume}. Like the zero APD restriction, this limit will apply until a

negative UPD is found downstream. It may also be sensible to limit road segment lengths to no more than 2-3 km to provide some additional precision in the results.

Figure 3.12 APD (Accrued Passing Demand) adjusted for bunching.

Running Distance (km)

Accrued Passing Demand (APD)

Two Options Converge Without Passing Lane With Passing Lane Slope = UPD per km APD is High => Demand is less => UPD is less APD is Low => Demand is higher => UPD is higher