DART's downtown street mall isn't as bad as some suggest. Many cities, not just Dallas, run light rail trains in city streets in central business districts. Most do so in streets shared with other traffic. At least DART runs the trains in streets reserved exclusively for trains. It's the cross streets that cause delays, and the delays aren't as long today as they used to be with the new priority traffic signals.
The train's max speeds downtown is the same as vehicles on the street would be, around 30 mph.
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There would be delays at the four station stops on the streetmall anyways, giving passengers plenty of time to safely get off and on the trains. Any additional wait at signal lights for cross street traffic is very minimal.
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It shouldn't be surprising that any double track light rail corridor is going to reach maximum capacity at some point, and DART will soon squeeze four light rail lines onto this single corridor, maximizing the corridor to its fullest extent. One may suggest that DART didn't design further expansion properly, one can also argue that DART design the system to just Phase II expansion capacity very efficiently, without any unused, left over capacity.
DART will not be able to afford more light rail corridors, or expansion of their existing corridors, after completing construction of the Phase II projects, for a long time to come (after 2030 --more than 20 years from now). At that time, DART may wish to build something else besides more light rail lines......
The next two new light rail corridors DART was planning, located in medians of city streets for a few miles both east and west. In both cases, streetcars may be the better choice than light rail trains. Who has a clear crystal ball and knows today what DART will do 20 years from now? I certainly don't.