reduce strain on the Central Area, several regional centres have been developed, each containing a concentrated commercial district. Light industry tends to be distributed around the island as industrial estates and located in flats, similar to the HDB, and only allow tenants which produce close to nil pollution, whereas heavy industry tends to be located around Jurong and Jurong Island. A list of such estates can be found here. There are two connections to the state of Johor, Malaysia. In the north there is a causeway for both rail and road traffic, which connects to the city of Johor Bahru. In the west there is a road bridge (Tuas Second Link), which connects to Johor, for road traffic only. The connections are an important economic link to Malaysia, which can be seen as a hinterland. The causeway (1,038 m in length) was designed by Coode, Fizmaurice, Wilson and Mitchell of Westminster and constructed by Topham, Jones & Railton Ltd of London. It was started in 1909 as a railway link by Johor State Railway to connect Johor Bahru to Singapore, then the administrative headquarters of British interests in Southeast Asia. Construction of the road section started in 1919 and completed in 1923. The causeway has caused ecological problems, most notably accumulation of silt in the Johore Strait. This has led to disagreements with Malaysia. Singapore has rejected Malaysia's proposal to replace the causeway with a bridge, and Malaysia has since proposed the idea of what became known as "the crooked half-bridge", descending halfway to link up with the low-level causeway. Since Singapore lacks natural freshwater rivers and lakes, the primary source of domestic water is rainfall. Demand for fresh water is approximately twice that supplied by rainfall, so Singapore imports much of its fresh water from Malaysia and Indonesia. To lessen its reliance on imports, Singapore has built reservoirs to collect rainwater from both forest catchment and urban catchment run-offs and from recycled water facilities. In addition, Singapore has built three desalination plants on the western coast of Tuas with two more under construction. They are expected to at least meet 30% of Singapore's water demands in 2060. Recent plans to produce NEWater-recycled water from treatment by filtration via reverse osmosis have been very successful as the government has opened up three plants around the island to help meet the demand. There are more than 300 parks and 4 nature