Transport Safety Blog NZ : Christchurch’s Dangerous Intersections [2]: Basic Principles

First published on Transport Safety Blog NZ

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[Please note: TSBNZ does not claim particular expertise in roading design. Hence posts on this subject are largely written from a longstanding interest in surface transport issues and practical observations of surface transport infrastructure design in large New Zealand cities.]

This post attempts to look at some basic issues of intersection design and how these can lead to safety problems over time. Intersections are one of the most significant components of a roading system and often require significant design effort and financial investment to ensure they contribute significantly to the safety and usability of roads, especially in urban environments. In TSBNZ observation, intersections work best when they are three or four way (tee or cross) and have regular dimensions and angles. Whilst any type of intersection can have issues or challenges, the simple types of intersections are the most straightforward in which to resolve these challenges. The most difficult intersections and where there tend to be higher accident risks are those where angles or dimensions are irregular, or where there are more than four directions of traffic, or where there are two intersections that are very close together, creating in effect a complex/irregular single intersection. TSBNZ believes this also holds true in the common situation of a railway level crossing being put onto one side of a road intersection, since it is clear a railway crossing is effectively its own four way intersection and placing it too close to any type of road intersection effectively creates a complex irregular intersection for traffic of all kinds.

The main issues that arise at any intersection are when traffic flows through any part of the intersection create a significant obstruction to another traffic flow. Generally, the key principle of any intersection is that some traffic is required to give way to other traffic. On a tee intersection, the traffic on the straight through route generally has a greater priority than traffic which is turning, either from the straight route to the side route, or vice versa. Traffic which has to turn across only one lane, such as left turning, generally is much safer than traffic crossing two lanes, such as right turning. However, any turning traffic can be obstructed by high traffic flows in any direction. On a cross intersection, one of the straight routes is often given greater priority than the other, and the lower priority route may have stop signs whereas the higher priority may not have any controls. The main risk arises when diverging traffic encounters, for example at busy times, long waits for a gap to open up in the straight flow, then people can get impatient and take risks, the risk being higher for people with lesser experience of driving, this is where accidents tend to occur. As traffic flows continue to increase in most cities over time, this problem multiplies, especially as secondary traffic routes with historically uncontrolled simple intersections become busier and there is an increased demand for safe provision for diverging traffic.

At this point there are various ways in which traffic flows can be managed at intersections. Simple sign controls are the cheapest management but are frequently inadequate since they do very little to address the challenges of a diverging flow crossing a dominant flow. Generally the imposition of physical structures into an intersection are the next highest form of intervention, beginning with various types of islands to direct or restrict traffic flows. Restricting divergent traffic to left turning only using islands to block right turning is one intervention seen in various places, but it is understandably less popular as motorists are forced to use other intersections nearby, and as these other intersections become busier, there will be pressure to reopen a restricted intersection. Islands are also used in some places to restrict traffic from causing safety issues by cutting across the wrong side of the intersection, and a lot of islands also double as crossing refuges for pedestrians.

A roundabout is a more expensive intervention, and can be seen at both tee and cross intersections. Roundabouts ideally should just have a single lane of traffic, and the intersection should not have more than four directions. Problems are generally seen more often at roundabouts with two (or more?) lanes of traffic (TSBNZ has yet to see any with more than two lanes in NZ) and those placed at intersections with more than four directions; these types are found at some of the most dangerous intersections in Christchurch. For this reason, it can be reasonably concluded that roundabouts in the latter category are an attempt by officials to push them beyond safe design parameters in difficult intersections with more than four directions of traffic or large volumes of traffic, including four lane highways, this being done mainly for fiscal reasons, not safety reasons. A small number of roundabouts in NZ have railway tracks running through the middle, the known examples being at Napier, Blenheim and Kumara; the Blenheim example, which is due to be removed after lobbying from disgruntled locals, is the bannerhead on TSBNZ’s Facebook page. Adding a railway line into a roundabout is effectively a complex and very irregular type of intersection and a novelty that has been foisted onto road users as a cheap but problematic solution for all types of traffic. Some of the issues of large roundabouts will be canvassed in subsequent parts of this series as most of the Christchurch examples appear in the top 100 dangerous intersections list.

Most expensive of all are signalised intersections. These are often the only safe way of protecting diverging flows of traffic movement, since a roundabout does not guarantee that at peak times there will be a safe enough gap for someone from a less busy side road to enter the traffic stream on the busy main road, hence accidents are still possible. Roundabouts are also of no benefit to pedestrians on very busy roads. They also require more land to install at intersections and where this is a consideration it is often simpler to signalise rather than consider a roundabout. Hence there are many sets of traffic signals in the centre of Christchurch due to the high flows through the city. At most ordinary intersections there are generally two basic phases of signals, one for each road that is intersecting, as diverging traffic is able to wait for a gap in the opposite flow in order to right turn. Right turn arrows may be provided at some intersections to make diverging safer where there is a high volume of straight-through traffic. It is becoming more common to see pedestrian protection provided for at these intersections with red arrows to prevent left turning traffic from proceeding when pedestrians have a “go” crossing signal. More complex signalised intersections may have a different phase for each traffic direction, if the traffic volume is very heavy, since this makes it easier for traffic to go in any direction without being blocked by opposing flow, where a lot of traffic tends to diverge. Signalisation is used in a number of places for less than a full traffic flow. There are number of signalised pedestrian crossings in places, as well as some for cycleways. An interesting variation is seen at the intersection of Wilsons Road and Charles Street on the Heathcote Express Cycleway where one set of lights effectively control both traffic flows in the intersection to provide a safe crossing for cyclists and pedestrians. Another variation is seen at the intersection of Aldwins Road and Harrow Street where signals stop only northbound through traffic on Aldwins Road to provide safe turning opportunities for right turners from both roads. Right turning vehicles from Harrow Street enter a separate lane in Aldwins Road and must then merge into existing traffic, whilst right turners from Aldwins Road are given their own separate lane, in both cases islands are used to separate controlled and uncontrolled flows. However this simplified signalisation is only achievable by omitting pedestrian facilities at this location.

The highest traffic volume intersections in NZ and elsewhere generally employ grade separation. This is typically required where a local road joins a motorway and typically provides for safe on and off ramps between the motorway and the local road, whilst provisioning the straight through traffic to cross directly over the top of the local road. There are also several examples of grade separation used on local roads in Christchurch. The intersection of Moorhouse Avenue and Colombo Street provides an overbridge for straight through flows on Moorhouse Avenue. Railway overbridges at various places around the city also provide for diverging roadways underneath next to the tracks.

Well hopefully that makes some sense and then the next part can take a look at the issues that arise for dangerous intersections in Christchurch.