Cul-de-Sac traffic system for DHA Phase VIII

[CMY]

By Shahzad Shah Jillani

KARACHI: The cul-de-sac or dead-end street is the preferred method of traffic control in DHA’s upcoming phase VIII with its residents choosing it over loop streets, Daily Times has learnt from a discussion between them and consultant engineers.

“Cul-de-sac” commonly refers to a street, lane, or passage closed at one end, a blind alley with no outlet except at the entrance, explained DHA Spokesman Col. (Retd) Raffat Naqvi. Other options for street design include the commonly used thoroughfares, which are open at opposite ends, and loop streets, which have two openings and the road connecting the two curves in the middle forms a semi-circle.

The cul-de-sac system has been widely used in Islamabad’s street design and many streets in the residential areas, particularly in the F, G and I sectors are cul-de-sacs.

He went on to say that since the early use of the cul-de-sac from 1928 as part of the hierarchical circulation system in the design of Radburn, New Jersey, this design has been chosen the most to control traffic. The town’s structure exemplified the ideal subdivision layout. Radburn was a town in which children need never dodge motor trucks on their way to school.

Residents have yet to completely assimilate the idea. While some said that cul-de-sacs were safer and quieter, other residents, including a professional engineer, opposed the design on various grounds. “It lacks the interconnectedness of development patterns like the gridiron, where streets intersect each other to form a grid pattern. One must always leave the cul-de-sac via a collector street to go anywhere, leaving minimal route choices, so one is stuck using the same path day after day,” claimed the engineer-resident.

He said that pedestrian travel will be long and boring with inefficient connections to nearby destinations. “One lacks the sense of being in a neighborhood or town with a civic identity. Main streets and tree-lined corridors that connect places and communicate the character and structure of a community are absent and what’s left is a string of dead ends on faceless connectors that lead nowhere,” he complained.

Another concern that DHA needs to address will be driveway for emergency vehicles such as fire brigades and ambulances, to provide a sense of security to the residents. According to Naqvi, suburban cul-de-sacs were previously short, straight streets with just a few houses. They were intended to provide a safe dwelling for residents allowing slow, smooth car movements to and from their houses. Today, with increased car ownership, the cul-de-sac has grown wider and much longer with more houses along it. “It ends in a circular space large enough for service and emergency vehicles to turn around, often more than a hundred feet in diameter. In its pure form, all the houses in a subdivision are situated on cul-de-sacs and as few as possible are placed on busier and noisier collector streets. A close cousin of the cul-de-sac is the loop street, which is similar in that it discourages traffic, going nowhere other than to the homes along it. However it has two access points and is usually longer than the cul-de-sac. Both loops and cul-de-sacs are often found in the same development,” he added. What is more convenient for emergency vehicles? Driving through main roads laden with heavy traffic, consuming extra time or using a street with clear traffic with a through exit?

Moreover, the cul-de-sac pattern has been strongly encouraged by traffic engineering and subdivision standards. “Ever since one of the first engineering studies on residential street safety was done in Los Angeles between 1951 and 1956, the Institute of Transportation Engineers (ITE) has recommended a hierarchical discontinuous street system for residential neighborhoods. The study showed that the number of accidents was substantially higher in grid-based subdivisions so the ITE established engineering standards using cul-de-sacs. The standards incorporated limited access to the perimeter highway, discontinuous local streets that discourage traffic, curvilinear design patterns, cul-de-sacs, short streets, elbow turns, T-intersections, and a clear distinction between access streets and neighborhood collectors,” informed a resident engineer of Osmani consultants.

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