Roads are a little like toilets. You might have a house with two toilets but, if you have a large family or a number of flatmates, both of the toilets might be occupied when you need to “go”.
At the point of your most need you think it would be a fine idea to, sometime soon, add another facility. Later, after the building work has been completed and now with three toilets, you think you’re bound to have enough evacuation-infrastructure to satisfy demand.
And most of the time you most certainly would. But then you order in, and consume, a dodgy vindaloo, shared with the whole household. This leads to simultaneous urges, and not even three toilets can now cope with the inevitable (bum) rush.
You realise that the only way you could guarantee a space on the privy council for every member of the household would be to have one porcelain throne per person. To build such facilities would require either converting some existing rooms or erecting an extension, but let’s say local planning laws prevent you from adding rooms to your house. In which case, for the first extra toilet, one of the spare bedrooms has to go the journey. For the second extra toilet you’re forced to reduce the size of the kitchen. But no worries, at least everybody in the household won’t have to squeeze their legs tight ever again.
Except you then throw a party, and fifty people show up, and they too are inflicted with food poisoning (very possibly because of your poor kitchen hygiene since downsizing it). All of a sudden not even five toilets are enough – in every spare corner of your house there are people with desperate need to get someplace shiny.
Naturally, as a kind host you never wish for such a faux pas to ever happen again and you partition your house until there’s no living space left – once spacious, it’s now merely commodious; every room is the littlest one. Flushed with success you know that, finally, you can meet every possible domestic sanitation eventuality. But, really, you’re just living in a sewer.
We have plenty of roads to go around, it’s when we all want to use the roads at the exact same time that’s the problem. Building more roads doesn’t fulfil demand, it just induces it.