Safety at the intersection
RAE FURLONGE Thursday, August 16 2012
TRAFFIC operations and safety are critical factors in the selection of intersection types.
Other factors include, land space requirements, environmental factors, operations and maintenance costs, and aesthetics. There are two types: at-grade and grade-separated. At-grade intersections are those where all vehicles travel within the space land space, and right-of-way is based on road regulation priority and time of entry; these types are all-stop, major-minor, traffic signal, and roundabout. Grade-separated intersections are those where vehicles are separated by levels, either above or below; these are also called interchanges.
Roundabouts are becoming popular again as a form of intersection control device, and this article discusses them. Major roundabouts still remain in TT after many years, some more than sixty years, and include the following, in no order of priority:
• Maritime Plaza Roundabout,
• Point Lisas Roundabout (also known as Brechin Castle (BC) Roundabout)
• Pointe-a-Pierre Roundabout at Petrotrin
• Marabella Roundabout,
• Mon Repos Roundabout,
• Rio Claro Roundabout,
• Sangre Grande Roundabout, and
• Roxy Roundabout.
Some major roundabouts have been removed over the years, and include:
• Curepe Roundabout at the intersection of Churchill-Roosevelt Highway and the Southern Main Road, removed in the 1980s,
• NP Roundabout at the intersection of Beetham Highway and NP Sea Lots, removed in the late 1980s,
• Chaguanas Roundabout, formerly at the location of the Chaguanas Flyover, removed during the construction of the Uriah Butler Highway (then called Princess Margaret Highway), and
• The roundabout at the former T-intersection of the Churchill-Roosevelt Highway and the Uriah Butler Highway (then called Princess Margaret Highway).
One of the largest roundabouts in the world was created by the unidirectional traffic operations of the Queen’s Park East, Queen’s Park West, Maraval Road, and Circular Road in POS, of the order of 5 km (commonly known as the Savannah). This was done by the Traffic Management Branch in the early 1980s. One of the planners for that major scheme was transportation engineer, Dr Trevor Townsend. At that time all the roads mentioned above were two-way, and most of their intersections were signal-controlled.
In recent times some major roundabout intersections have been created, and include the following, in no order of priority:
• Piarco Airport Roundabout,
• Trincity Roundabout,
• Couva Main Road Roundabout and Rivulet Road Roundabout in Preysal,
• Price Plaza Roundabout in Chaguanas, and
• Scarborough Hospital Roundabout in Tobago.
Some supporters of the current traffic scheme for Woodbrook and St James are comparing that scheme with the revolutionary Queen’s Park Savannah Roundabout scheme (discussed above), and are claiming that the earlier project was subject to many criticisms, and who could imagine the Queen’s Park Savannah without the one-way system. They are therefore demanding patience, so that the new project could be eventually accepted.
But, there are fundamental differences between both schemes:
1. The intersections along the Queen’s Park Savannah do not require turning movements into the Savannah, and so these are all T-intersection, which would lend themselves naturally to clockwise circulatory movement.
2. The roads around the Savannah are generally three-lane primary arterial roads, or main transport routes within the road hierarchy, and function with limited frontage access, minimal number of intersections, pedestrian movements clearly segregated, and signalised pedestrian crossings wherever possible, minimal parking permitted on the road and, no stopping on the roadway.
Tragarete Road, a two-lane, two-way secondary arterial road, and Western Main Road, a four-lane, two-way undivided urban arterial road, where there is a relaxation of the direct access and pedestrian constraints, to differentiate them from primary arterial roads.
Ariapita Avenue is collector road, which means that the needs of moving traffic still predominate but they also contribute to access requirements. Collector roads serve to feed traffic onto and off the arterial roads. They operate under the following conditions: motorists need to be aware of pedestrians as these roads will be within or close to residential areas; the road is only for local traffic and through-movements should be made awkward and inconvenient to discourage them; vehicle speeds should be kept low; parking may be allowed, but alternative off-road provision is preferable; and public transport stops may be located on the carriageway but should be near well-defined crossings.
Roxy Roundabout appears to have created problems for the current scheme. Roxy Roundabout is elliptical in shape, with four legs. The eastern leg is Tragarete Road, the western leg is Western Main Road and Serpentine Road merger, the northern leg is St Clair Avenue, and the southern leg is Damien Street. The ellipse is placed at a skewed angle to the Western Main Road / Tragarete Road alignment.
On the eastern side of Roxy Roundabout, Tragarete Road is about 14 m wide, but due to single (centreline) roadmarking and parking, operates as a two-lane, two-way arterial road. On the western side of Roxy Roundabout, Western Main Road is also 14 m wide, but roadmarking delineation provides four-lane, two-way traffic.
Damien Street is a two-lane two-way collector road, in that it provides for movement between the arterial roads of Western Main Road, Wrightson Road and Audrey Jeffers Highway, and the local roads in the community. It serves an arterial road function as well, in that through-traffic between the arterial roads above is accommodated. In fact, that is currently the primary function. It averages about 7 m wide, with a paved sidewalk on both sides. Sight distance is generally good.
St Clair Avenue is a two-lane, two-way collector road connecting the arterial roads of Queen’s Park West and Maraval Road in the east, with Western Main Road and Tragarete Road, and the local roads in the community. It averages about 9 m wide, with paved sidewalks on both sides. Sight distance is generally good.
Before the scheme, traffic operations at Roxy Roundabout during the weekday evening peak period were severely affected in the weaving section from Damien Street to Western Main Road, with average speed 14.2 km/h, and the weaving section from Tragarete Road to Damien Street, with average speed 24.1 km/h.
To be continued next week.