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INTERSECTION CONTROLS

signals at such intersections result in high delays to the through movements. They are also appropriate at ‘T’ or ‘Y’ intersections where the major traffic flow turns through a right angle, and multi-approach intersections. Roundabouts may not be successful where traffic volumes are unbalanced where one approach has significantly more traffic than the other approaches. Roundabouts are also not successful in areas of high pedestrian activity and care needs to be taken in areas of high cycle activity. During off peak periods, delays are often less than traffic signals, as vehicles are never faced with a “red light”.

A single lane roundabout has the capacity to carry around 3,000 to 3,500 vehicles per hour.

20.3 SIGNALISED INTERSECTIONS

Signalised intersections control traffic by allowing opposing vehicles to travel at different times.

The effectiveness of signals depends upon the nature of their design, the adequacy of maintenance and the correct interpretation and timely reaction by road users.

The capacity of the intersection approach needs to be greater than the capacity downstream of the signals to accommodate the phases during which traffic is stopped. Additional phases, such as right turn phases, further encroach in the amount of green time available for an approach, often requiring multi-lane approaches. Often the extra lanes can be accommodated by removing adjacent parking, however with high traffic volumes, the carriageway will often require widening, to get enough capacity at an intersection. Generally the capacity of a signal intersection is higher than a roundabout, although if the proportion of right turning vehicles is high and the traffic flow is equally distributed amongst the approaches, a roundabout can sometimes be more efficient.

20.4 ACCIDENT RATES

Typical intersection accident rates vary in accordance with the type of control and are shown in Table 21.

Table 21: Intersection Accident Rates

TYPE OF INTERSECTION INJURY ACCIDENT RATE

(ACCIDENTS PER 1,000,000 VEHICLES)

Tee Intersections Priority 0.068 Traffic Signal 0.042 Cross Intersections Priority 0.124 Roundabout 0.115 Signals 0.134

20.5 INTERSECTION DELAY

Urban traffic congestion is primarily a result of intersection delay. The difference in travel time between peak periods and non-peak periods is due to the increased pressure at intersections.

The average delay to motorists at an intersection is a key factor in determining whether an intersection control requires upgrading or altering. In some areas, high intersection delays are acceptable, as a means of reducing through traffic, eg local roads and parking streets. A tiered level of acceptable delays therefore needs to be established. Peak delays over a short timeframe does not warrant additional lanes at an intersection if the intersection is going to be well within capacity for the remainder of the day. The following levels of delay are the recommended maximum, prior to a roading improvement being required.

Arterial Streets - average delay of 25 seconds per vehicle for all approaches over a continuous period of two hours.

Collector Roads - average delay of 30 seconds per vehicle for all approaches that are controlled over a continuous period of two hours

Local Roads - average delay of 40 seconds per vehicle for all approaches that are controlled over a continuous period of two hours.

Peak delays would be expected to be up to 50% higher over a 10 to 15 minute period.

20.6 PALMERSTON NORTH INTERSECTIONS

Presently Palmerston North has 28 signalised intersections and 15 roundabout controlled intersections. Analysis of the projected future turning movements at these intersections show that the delays at many of the intersections become excessive, even though the total approach flows are within the capacity ranges. A closer examination shows that the proportion of right turning traffic at many signalised intersections are high, and the flows could be better handled by roundabout control.

Appendix 1 in the Supporting Information document shows the approach volumes and percent of right turning volumes at various intersections within Palmerston North. The flows shown have been surveyed within the previous 18 months.

A high proportion of intersections have greater than 15% right turning vehicles. However some of these intersections have much heavier traffic volumes on some approaches, and accordingly would not be suitable for roundabouts, eg Tremaine/Rangitikei and Te Awe Awe/Fitzherbert. Further particular investigation and design is needed.

As the arterial road network becomes increasingly busy, through traffic intrudes onto local roads. This intrusion can spoil a previously quiet neighbourhood prompting local communities to look for relief, generally in the form of speed humps, chicanes, road narrowing and roundabouts. In order to achieve this consistently throughout the city, Council has developed a Local Area Traffic Management Policy and Guidelines. For a scheme to work and be effective, it must have community support. In this respect, the established policy only allows traffic calming to occur on local and collector roads in areas where agreement is obtained from at least 75% of the adjacent land owners, as the traffic calming may reduce on-street parking, increase traffic noise and increase local travel times.

Campbell Street is an example where local area traffic management (LATM) has been used to effectively remove through traffic from a residential street.

Less severe traffic management can be achieved by extending a footpath across a side road such that the side road looks more like a driveway than a street to the passing motorist. This technique is used extensively in other cities around New Zealand.

Care needs to be taken in areas that have LATM and a high use of cyclists, particularly at kerb extensions and raised thresholds.

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