It's a lot more complicated than that, if I remember my research on this subject correctly. It is definitely true that in a perfectly abstract system that if your roads are 70% utilized across the board, and you double capacity, you won't stay at 70% capacity.
However, what does happen is that when you take a road at 70% capacity and double it, so it has a lot more room, some people will move from other 70% utilized roads to your road, and yet more people will decide to drive instead of taking the bus or train, and the utilization of the whole system will remain largely flat at ~70%.
This was seen time and time again in NYC, where even at the peak of construction of roadways in and out of the city under Robert Moses, every new bridge, tunnel, and highway was at >80% utilization within days or weeks of completion. IIRC, the total amount of major roadways in and around NYC was increased by 5-10x from the 1940s to 1950s, and yet traffic congestion and utilization actually increased over this time.
It took a number of years of urban planning research to identify that as you increase the number of roads, more people opt to drive in lieu of mass transit or not making the trip at all.
So, i would argue that in an abstract sense you are correct, but in the context of a major city, a single major road change does not ease congestion, as it simply changes traffic flows and increases demand.
% capacity and % utilization are measures of congestion - not of demand. If your roads are only meeting 10% of latent demand and you double their capacity, guess what? They will be just as congested as before.
Leave it to "years of urban planning" to determine a proper solution is to make roads so poor that people decide not to use them.
> Leave it to "years of urban planning" to determine a proper solution is to make roads so poor that people decide not to use them.
What a weird level of snark. At least in NYC, demand is effectively infinite. Close to 2 million people commute into Manhattan every day, not counting commercial vehicle transportation. Building enough roads to satisfy all of those people is theoretically possible, but leads to plans like these:
I can't find it now, unfortunately, but there was a serious analysis done of what would be required to accommodate all of the potential demand for car transit into Manhattan, and it was insane how many bridges and roadways were required.
Thats' why you can't look purely at demand. As you make it easier to drive into a city, more people will opt to do so, thereby increasing both demand and capacity.
You even see this in US cities like Atlanta that are not islands, where increasing demand led to more and more road construction, which drove more and more people to live further and further outside the city, since they could just drive an extra 15-20 minutes each day. In the end, the only thing that capped demand for Atlanta was rising gas prices, making it more expensive to commute.
That's why you talk about capacity and utilization - demand is effectively infinite until other costs and considerations reduce demand.
NYC is the most extreme case. A good local example of is the San Mateo Bridge, at rush hour the San Mateo Bridge / 92 interchange backs up the 101 all the way to Woodside (several miles) but clears up after that. By the time they get around to expanding the bridge or adding another one (ha, when pigs fly) the demand will cause whatever the replacement is to be jammed as well as I would assume many more people would live in the East Bay which is cheaper.
I guess we would have to do something that requires effort like actually examine why people need to drive at all. The real problem is probably workers being denied the opportunity to live close to their places of employment due to predatory pricing and poor services downtown. But then we couldn't put the blame the average working stiff or use it as an excuse to decrease public works spending.
However, what does happen is that when you take a road at 70% capacity and double it, so it has a lot more room, some people will move from other 70% utilized roads to your road, and yet more people will decide to drive instead of taking the bus or train, and the utilization of the whole system will remain largely flat at ~70%.
This was seen time and time again in NYC, where even at the peak of construction of roadways in and out of the city under Robert Moses, every new bridge, tunnel, and highway was at >80% utilization within days or weeks of completion. IIRC, the total amount of major roadways in and around NYC was increased by 5-10x from the 1940s to 1950s, and yet traffic congestion and utilization actually increased over this time.
It took a number of years of urban planning research to identify that as you increase the number of roads, more people opt to drive in lieu of mass transit or not making the trip at all.
So, i would argue that in an abstract sense you are correct, but in the context of a major city, a single major road change does not ease congestion, as it simply changes traffic flows and increases demand.