To soapbox a bit on VMT: VMT is not a measure of transportation demand. It's a measure of transportation consumption, like gasoline. Vehicle miles traveled means mileage on your car, time lost in congestion, and wear and tear on the road.
It's been used for a long time because it's an easy metric to interpret. You invest in places where that investment is likely to be used. But while that make sense when you are the Bureau of Public Roads in 1956, trying to decide which highway to pave next, it's less useful today.
Because most of the VMT today isn't on rural highways. It's on urban roads. And urban roads are different than rural highways--they aren't 'corridors' but networks (pfah to 'corridor studies' everywhere). All the roads interconnect, and there are multiple pathways between two different points. And in a network, ideally, you want people to take the shortest point between two routes. Because every mile on the network is road capacity consumed, and pollution generated. But that never happens, because we have freeways, which are much much faster than surface streets. And so there is always the temptation to detour to the freeway, drive along the freeway, and then drive back to where you wanted to go. And which you pick all depends on how fast the freeway is compared to the surface streets.
There is an entire field of modeling, known as transportation demand modeling, designed to overcome the limitations of corridor studies. Millions of dollars every year go into developing, maintaining and using these models, designed to answer the question of WHICH corridor traffic will use to get between two points. (Or combination of corridors, or combination of streets, or series of network segments, etc). All in order to best predict which route is going to be most crowded in 10 years, so they can get a jump on things.
There is a fundamental shift there--from providing connectivity between places (rural highways) to make places more accessible to one another, to ensuring mobility between places (urban highways).
Here is the fundamental of urban economics: There is an equilibrium between land costs and transportation costs. Anything you do to change the cost of land or the cost of transportation shifts that equilibrium. But nothing can change the fact that there is an equilibrium. Lower transport costs, and you lower land costs. But as transport costs rise (traffic, tolls, etc), land costs rise as well.
But here is the rub in urban transportation: The cost of each increment of speed improvement isn't linear. Every increment costs more. The second lane costs more the first. The 8th lane costs more than the 4th. Because the first increment is always built using the easiest/cheapest way. So that's used up, and the second increment has to make do with the second best bit of right of way: The part that requires demolishing a whole house, digging deeper into the edge of the hill.
But I've strayed from the topic at hand: Soapboxing about VMT. VMT is a cost of travel. For corridors, it's a reasonable way to allocate funds. For urban networks, it's silly: The most VMT doesn't necessarily indicate the best (most direct) route between places--just the fastest. And so we keep dumping money into congested routes, hoping they will get better. But they never do. The fastest route is still the fastest route, and if you make it a little bit better, more people use it. The scientific name for it is 'induced demand', as people divert from other corridors, other times, and other modes of travel.
Now, let's talk about urban area VMT? Is SLC the best, if we have the most VMT? Or the most VMT per capita? Or the most VMT/GDP? NO. Exactly, the opposite, actually. VMT/capita is a measure of how much the average person HAS to drive (HAS to pay) to get done the things they want to do. And VMT/GDP is a a measure of how much transportation (as an economic input) that has to be consumed per unit of GDP. It's like hours of labor: The amount of resource that must be consumed per unit of output. Economists have a term for that: "Labor productivity". How much labor is required to make a dollar worth of value? HINT: a high number is not a good thing--it's the sort of thing that characterizes sub-Saharan Africa, or under-industrialized countries. They lack all the things that makes labor efficient: Tools, machines, training, education. So high VMT/GDP is a bad thing. It means the regional economy (the economy of the urbanized area) is inefficient.
So when the DOT sees VMT on a road going up, they feel like they should dump some money there. But when you add up all the VMT on all the roads, that's a cost. So when the DOT dumps money where there is VMT (to create more VMT), it's not actually a good thing. (Nor does it actually make your commuter any shorter--remember that the best/fastest route is still the best/fastest route, and if you make it faster, more people will take it, until it gets worse, and is no longer the best route).
simple metric: What's a good place to put transportation dollars? Where people
It's been used for a long time because it's an easy metric to interpret. You invest in places where that investment is likely to be used. But while that make sense when you are the Bureau of Public Roads in 1956, trying to decide which highway to pave next, it's less useful today.
Because most of the VMT today isn't on rural highways. It's on urban roads. And urban roads are different than rural highways--they aren't 'corridors' but networks (pfah to 'corridor studies' everywhere). All the roads interconnect, and there are multiple pathways between two different points. And in a network, ideally, you want people to take the shortest point between two routes. Because every mile on the network is road capacity consumed, and pollution generated. But that never happens, because we have freeways, which are much much faster than surface streets. And so there is always the temptation to detour to the freeway, drive along the freeway, and then drive back to where you wanted to go. And which you pick all depends on how fast the freeway is compared to the surface streets.
There is an entire field of modeling, known as transportation demand modeling, designed to overcome the limitations of corridor studies. Millions of dollars every year go into developing, maintaining and using these models, designed to answer the question of WHICH corridor traffic will use to get between two points. (Or combination of corridors, or combination of streets, or series of network segments, etc). All in order to best predict which route is going to be most crowded in 10 years, so they can get a jump on things.
There is a fundamental shift there--from providing connectivity between places (rural highways) to make places more accessible to one another, to ensuring mobility between places (urban highways).
Here is the fundamental of urban economics: There is an equilibrium between land costs and transportation costs. Anything you do to change the cost of land or the cost of transportation shifts that equilibrium. But nothing can change the fact that there is an equilibrium. Lower transport costs, and you lower land costs. But as transport costs rise (traffic, tolls, etc), land costs rise as well.
But here is the rub in urban transportation: The cost of each increment of speed improvement isn't linear. Every increment costs more. The second lane costs more the first. The 8th lane costs more than the 4th. Because the first increment is always built using the easiest/cheapest way. So that's used up, and the second increment has to make do with the second best bit of right of way: The part that requires demolishing a whole house, digging deeper into the edge of the hill.
But I've strayed from the topic at hand: Soapboxing about VMT. VMT is a cost of travel. For corridors, it's a reasonable way to allocate funds. For urban networks, it's silly: The most VMT doesn't necessarily indicate the best (most direct) route between places--just the fastest. And so we keep dumping money into congested routes, hoping they will get better. But they never do. The fastest route is still the fastest route, and if you make it a little bit better, more people use it. The scientific name for it is 'induced demand', as people divert from other corridors, other times, and other modes of travel.
Now, let's talk about urban area VMT? Is SLC the best, if we have the most VMT? Or the most VMT per capita? Or the most VMT/GDP? NO. Exactly, the opposite, actually. VMT/capita is a measure of how much the average person HAS to drive (HAS to pay) to get done the things they want to do. And VMT/GDP is a a measure of how much transportation (as an economic input) that has to be consumed per unit of GDP. It's like hours of labor: The amount of resource that must be consumed per unit of output. Economists have a term for that: "Labor productivity". How much labor is required to make a dollar worth of value? HINT: a high number is not a good thing--it's the sort of thing that characterizes sub-Saharan Africa, or under-industrialized countries. They lack all the things that makes labor efficient: Tools, machines, training, education. So high VMT/GDP is a bad thing. It means the regional economy (the economy of the urbanized area) is inefficient.
So when the DOT sees VMT on a road going up, they feel like they should dump some money there. But when you add up all the VMT on all the roads, that's a cost. So when the DOT dumps money where there is VMT (to create more VMT), it's not actually a good thing. (Nor does it actually make your commuter any shorter--remember that the best/fastest route is still the best/fastest route, and if you make it faster, more people will take it, until it gets worse, and is no longer the best route).
simple metric: What's a good place to put transportation dollars? Where people
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