LOCAL AREA TRAFFIC MANAGEMENT

The use of local area traffic management (LATM) measures to produce safety and amenity improvements may occur at four levels: malls, residential precincts, corridors, and town-wide. This section deals with the use of LATM measures within the existing street system; i.e. at the first two levels. The other two levels are discussed in Chapters 30 & 31. The traffic aspects of LATM are examined further in Section 30.7.

LATM is sometimes also called traffic calming, but that term is best used to describe the creation of ‘peaceful coexistence’ between the mix of transport modes that the community wishes to see operating in a particular area. Traffic calming may therefore include both physical and non-physical measures such as road pricing (Section 29.4.6), the alleviation of the adverse effects of car and truck, and favoured treatment for public transport.

Many LATM measures are directly concerned with reducing vehicle speed and systematic, well-planned treatments can reduce local crashes by as much as 40 percent. However, LATM measures also have their disadvantages:

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* it is important to consider the area-wide effect of the installation of each device, as they will rarely eliminate traffic. Instead, they will commonly divert traffic to other routes and the consequences of this diversion on those routes must be considered. * there will often be an increase in average trip times, although this should not be of

concern. Given an average residential precinct trip of about 750 m, a drop in speed from 60 to 50 km/h would add only 9 s to a trip within the precinct. Similarly, most devices would delay a worst-case large vehicle by 10 s or less.

* there may be inconvenience to large vehicles and care must be taken to continue to provide access for:

– emergency services, – delivery vehicles, – garbage collection, – street-cleaning devices,

– public transport vehicles on defined routes. Some precinct-wide LATM measures are:

(a) One-way streets. Whilst these can reduce through traffic, they usually have no net effect on safety. They are appropriate for areas having narrow or parallel streets that are frequently congested. In these situations, a change to a one-way system can increase both capacity and safety. However, such schemes usually inconvenience local traffic and create strong local opposition.

(b) Use of T- rather than X-intersections, to reduce through movements (Section 20.3.2).

(c) Bans on heavy vehicles, however some heavy vehicles will be found to be servicing facilities within the precinct and alternative delivery procedures will need to be provided.

(d) Parking controls (Section 30.6.2).

Localised LATM measures can be categorised as street closures, intersection treatments, and mid-block treatments.

(1) Street closures are LATM measures aimed at deliberately reducing street connectivity. The common forms are:

(a) Half closures (or chokers) at one end of a street. These are rarely effective for streets over 300 m in length.

(b) Complete closures of one arm of an intersection.

(c) Complete closures at one end of a street, creating a cul-de-sac.

(d) Diagonal closures at intersections. These can reduce speeds at the intersection to 25 km/h (Section 18.2.5) but are not very effective in reducing midblock speeds.

(e) Midblock closures. These serve only to reduce local connectivity of the street network, they will have a lesser effect on intersection safety.

Closure of an existing street will initially cause local traffic disruption, but common experience is that traffic soon readjusts and there is a net reduction in traffic as some drivers seek travel alternatives (the reverse of traffic induction discussed in Section 31.3.3).

(2) Intersection design is discussed in Section 20.3. The common forms of inter- section treatment used in LATM are:

(a) Partitioning of intersections to only permit turning movements – these may be the same as a diagonal street-closure or they may use a star-shaped diverter at the centre of the intersection.

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(c) Small roundabouts. By their geometry, these force drivers to deviate from a straight line, and therefore to slow down, when passing through an intersection. Casualty plus reported damage accidents at all classes of road are reported to have dropped by about 50 percent and at minor intersections by 90 percent with the installation of such roundabouts (Ashton and Brindle, 1982). Large roundabouts are discussed in Section 20.3.6.

(d) Signing measures at intersections, particularly Give Way and Stop signs (Section 20.2.2) and turn bans, and reduced green times at traffic signals (Section 23.2.3).

(e) Intersection channelisation – islands at intersections within the street length often operate almost as small roundabouts (McKelvey and Thomas, 1984). (f) Plateaus (or speed tables) of raised pavement (Section 18.1.4) at an

intersection, to both slow traffic and to also highlight the intersection and its pedestrian movements. As the intersection already requires drivers to slow down, a plateau can have minimal influence on capacity.

(g) Pavement narrowings at intersections, which in the extreme reduce to driveway or threshold entries to the street. When all four approaches to an intersection are narrowed, the result is a Catherine wheel intersection.

(h) Divider (or splitter) islands placed along the centreline to denote the entry to a residential precinct (Section 7.2.3).

(i) Small kerb radii at the corners to prevent high-speed turns from one street to the other and to aid pedestrians. Speeds are reduced to about 25 km/h (Section 18.2.5).

(3) Common midblock LATM treatments are:

(a) Short lengths of pavement narrowing can reduce speeds by up to 10 percent and can also prove effective in most aspects of traffic management (Daff and Siggins, 1982).

(b) Techniques which cause the horizontal deflection of the traffic path are particularly effective. The pavement may be narrowed on either one or both sides of the street. Such treatments are also called chicanes, midblock restrictions, pinch points, throttles, angled slow points, or neckings and can reduce speeds by up to 30 percent locally and 15 percent in the local road network. Long lengths of pavement narrowing can also be used to produce a meandering or wandering route — this may additionally be used for aesthetic reasons.

(c) Staggered or meandering traffic lanes within a wide pavement, to produce an effect similar to (b) at lower cost.

(d) Pavement narrowing to increase footpath width and thus reduce the potential exposure of pedestrians (particularly children) to traffic.

(e) Narrowed traffic lanes to reduce traffic speed (Section 16.5.1 & 18.2.1), and possibly provide extra space for cyclists (Section 19.6).

(f) Divider islands along the centreline at midblock to slow traffic.

(g) Median strips to narrow pavements and provide pedestrian refuges (Section 20.4).

(h) Kerbline or median fencing to discourage unsafe pedestrian movements. (i) Rough surfaces or block paving to slow traffic (Section 18.1.4).

(j) Road humps, plateaux, ramps, and rumble areas to slow traffic, make the street context very apparent to the driver, deter through traffic, and improve safety

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(Section 18.1.4). Spoon drains (Section 13.3.2) have also been used but are not recommended.

(k) Tree planting, which is an effective speed-reducer through its effect on drivers’ perceptions of an area (Section 18.3).

7.4 THE STREET ITSELF