Secretariat, Taipei City Government

The density of bus routes in Taipei City is ranked top in the country, and to prevent large buses from affecting traffic safety and flow due to weaving caused by frequent roadside stoppages, most of the multi-lane thoroughfares in downtown Taipei feature dedicated bus lanes to alleviate traffic congestion during rush hours. However, since the dedicated bus lanes are only open to heavy buses, after prolonged use the asphalt concrete (AC) paving is prone to rutting because of the buses’ identical wheel path. This problem is compounded by the corrosion and softening of asphalt caused by the summer heat and spillage of unknown oils. As a result, road surface cracking is accelerated, in turn reducing the lifespan of the paving and increasing the cost and difficulty of road maintenance. On a more serious note, the cracks will also lower the safety and comfort level of public transport.

In an attempt to rectify the common flaw, after brainstorming, reviewing and assessing various considerations, the New Construction Office of the Public Works Department (NCO) decided to adopt a better pavement material known as stone matrix asphalt (SMA), which has exceptional rutting resistance and load-bearing performance. For the pilot program, the pavement will be applied to the southbound dedicated bus lane along Section 1 of Dunhua South Road (the section between Bade Road and Civic Boulevard), measuring approximately 250m. The pilot program is expected to generate noticeable improvements.

According to NCO Road Maintenance Section Chief Wang Chih-Liang, SMA was developed in Germany in the 1960s and used for paving high load-bearing roads, airports, and docks across Europe. It was introduced to North America in the 1990s, and Taiwan incorporated SMA into the Public Construction Outline Specifications more than a decade ago. The uniqueness of SMA is its coarse stone skeleton complemented by high filler and bitumen content as well as low fine aggregate content; the mastic provides binding and prevents water from entering the pavement. Compared to traditional dense graded asphalt concrete (DGAC), SMA has significantly larger aggregates, coarser texture, and denser filling to prevent water from entering the pavement, which means that water vapor is less likely to form during rain. Furthermore, the interlocking mechanism of the coarse stone skeleton increases the rutting-resistance of the pavement. However, SMA’s strength, durability, slip-resistance, low-noise, and flatness properties come at a much higher cost and require more sophisticated construction technology. Small- or large-scale applications of SMA pavement can be seen in domestic airport runways, national freeways, and roads around dense industrial zones.

NCO Public Works Section Central District Branch Director Chang Hao-Hsuan added that SMA is very thick and sticky, hence the temperature of the construction site may not be below 15℃: Otherwise, it will lose its properties as it congeals due to a rapid drop in temperature, making it difficult to pave. When paving with SMA, the temperature must be kept above 170℃. Meanwhile, other requirements and limitations apply during the mixing and rolling process, therefore the production process of the bitumen factory and procedure at the construction site must be planned meticulously to prevent failure.

The reason for choosing the southbound dedicated bus lane along Sec. 1 of Dunhua South Road (section between Bade Road and Civic Boulevard) to conduct the pilot program at night is because it is plagued by a number of serious ruts and that Dunhua South Road is relatively wider, thereby minimizing the impact on the surrounding traffic and the local residents’ lives. As a result, the construction workers will be able to concentrate on their work without being interrupted. The NT$900,000 pilot program was implemented at 22:00 on December 9 last year, and it was completed by 03:00 on December 10, paving 250m of road that measures one lane wide. The construction process went without a hitch, and the project’s subsequent benefits will be monitored closely.

Although the unit cost of the first SMA pavement project is relatively high, if the project generates good benefits within the short term, the NCO will continue to conduct pilot programs elsewhere. In the long run, if SMA pavement is able to eliminate rutting and significantly decrease maintenance frequency, the NCO does not rule out the possibility of implementing SMA pavement to all dedicated bus lanes in Taipei City in order to achieve the double objective of enhancing public transport service quality and saving public expenditure on maintenance.