More asphalt doesn't reduce road congestion .....

.... but light rail does.

From Transport & Environment (T & E)

We will have to get used to the fact that traffic jams are here to stay. After all, if traffic can move one per cent faster during rush hour, this will increase car commuting by just over one per cent. This follows from research by Rijkswaterstaat, the Dutch Road Authority. The study provides a simple insight into the behavioural effects of various mobility measures. These are average effects that can vary in specific situations.

Rijkswaterstaat's findings are in line with international research. A synthesis of five studies concludes that in urban areas, one per cent extra motorway capacity leads to one per cent extra traffic. For urban trunk roads, the figure is 0.75 percent. This confirms the 'fundamental law of road congestion', as one of the five underlying studies is called. A study of the impact of 16 motorway widenings in the Netherlands also shows that the additional traffic growth on the main road network exceeds the increase in capacity.

Congestion regulates mobility behaviour. Excessive congestion encourages drivers to choose a shorter journey, to avoid the rush hour, to travel with someone else or to use another mode of transport. Less congestion has the opposite effect. After the opening of the Zeeburger tunnel near Amsterdam, 16% more cars crossed the North Sea Canal during rush hour, the average occupancy of these cars fell by 7% and 31% of drivers returned to driving during rush hour.

So, congestion remains. Back in 1988, McKinsey advised in its report 'Ending traffic jams' that more asphalt would not solve congestion. The statistics bear this out: after some ups and downs, the time lost per kilometre driven on Dutch motorways today is about the same as it was twenty years ago. Traffic jams slow traffic down by an average of one tenth of a kilometre per hour.

This analysis also implies that converting roadways to pedestrian zones, i.e., making roads traffic-free, will not increase congestion, even while it makes using the streets more pleasant for pedestrians.

Meanwhile, building light rail leads to less congestion, shorter travel times, and less pollution: 

From Science Direct

I find evidence that an increase in the supply of [light]rails [in European cities] leads to less congestion, less travel time and less pollution. Furthermore, I find that cities with a new rail system have 7% less congestion, 1% less travel time and 3% less pollution than cities with no rail systems. I find no evidence of different pre-trends of treated and control cities. The impact of rails on congestion, travel time and pollution is gradual, and increases over time.

The analysis would imply that expanding a light-train or tram system after the first section has been constructed will lead to further traffic/congestion reductions, especially with network effects.

This paper suggests that in Melbourne (with the world's largest tram network) the impact is somewhat larger, despite stops being much closer together than on most light rail systems:

In inner Melbourne trams have a much higher impact in reducing congestion; vehicle time travelled and total delay on the road network decreases by 3.4% as a result of tram operations. The average road network speed rises from 41.6 km/h to 41.9 km/h (an increase of 0.9%). The operation of trams in inner Melbourne increases actual travel time on average from 2.14 minutes/km to 2.13 minutes/km. Although trams contribute to reduce the number of car trips on the road network, the average travel speed increases slightly. This is because the travel speed on links with non-exclusive tram rights-of-way decreases due to the low speed of trams and boarding/ alighting passengers. The tram network contributes to reduce 16% of the number of moderately congested links in inner Melbourne

The moral of the story is clear:  if your freeways are getting congested, don't add more lanes or more freeways.  Build light rail.  And add pedestrianised streets, which, the Melbourne experience shows, work particularly well with trams.

See also :  Metcalfe's law