Transportation Demand Management, often known as TDM, is an approach to transportation planning which generally seeks to influence travel behavior by changing the "demand" for driving. TDM is the type of program that planners (and consultants) love--it sounds good, gets the public excited, it's paid for by someone else, and is often innocuous enough to not do anything that makes anyone angry (or have any real effect). TDM Strategies often include a mixture of:
Subsidy programs ("Ride your bike to work and get a coupon/prize")
Apps ("Find carpoolers in your area" "Simplified purchasing of bus passes")
Marketing campaigns ("There's less pollution if you don't drive")
Bike share/Car Share
Showers/Changing rooms at work for bicycle commuters
Bicycling safety classes
Monetary incentives to switch modes
Parking pricing/congestion pricing
Many of these are fine, but they are mostly "carrots" rather than "sticks"--and in facilitating real change, the majority of them are just not that effective. Think back to your own experience--have any of the items above ever significantly influenced you to change your travel behavior? Would they be effective on your colleagues?
The only items from the lists above which really produce any results are the ones that address transportation supply: pricing roadways, reallocating right-of-way space, and pricing vehicle storage space (aka parking). This is because when the supply of a certain type of travel like is cheap and convenient, people consume more of it. When the supply of that same type of travel becomes more expensive or less convenient, people use less of it. Showers for bicyclists, marketing campaigns about clean air, and apps which show when your bus will arrive just do not materially impact the decision making process of the person who lives far from work and drives in every day because they don't change the factors that make driving convenient. Change the cost of a driver's commute, however, and you will probably see results (if the higher prices persist in the long term). That's why Transportation Demand Management should really just be Transportation Supply Management: focus on transportation supply like roads and parking, and cities can leave the other pieces to whomever.
Proponents of TDM swear it can make a difference, but it's hard to find any conclusive data which shows that any of the incentives above actually do anything. In my experience, it's also difficult to parse out which aspects of TDM have been effective when several have been implemented at once. This report from Massachusetts provides an overview of programs but no clear information about whether they were successful or not. This report waffles about how "It is difficult to evaluate the case studies collectively because so many disparate factors affect mode split."--but it also states clearly:
Parking, which raises the cost of SOV [single occupancy vehicle] commuting directly, affects mode choice much more than does any other factor. In fact, parking costs and SOVrates, are, by and large, inversely related. The charge must be quite high to reduce SOV rates.
This more recent report out of California suggests something similar. I am picking and choosing my sources based on what can be easily located online here, but I think in general most people understand: making driving less convenient/more expensive is a much stronger variable than some ad campaign about pollution or a weak incentive to ride a bicycle.
A good real-world comparison might be charging fees for plastic bags at the grocery store: Decades ago, if you brought your own bag to the grocery store they either did nothing or might take 5-10 cents off your total--as I kid I don't think I ever saw my parents bring reusable bags when shopping. Contrast that with how over the past 20 years or so, numerous cities have banned plastic bags or instituted a fee for using them, and this appears to have changed consumer behavior to use fewer plastic bags when leaving the grocery store. A minor cost seems to have been much more effective than a minor reward.
For another example: years ago I worked at a large planning & engineering firm which proudly offered a TDM "benefit" to eligible employees who biked to work: a $20 check available four times a year that could only be used at a bicycle shop in the area. There were no similar benefits for the people who walked to work or rode the subway, and no one seemed to notice any inconsistency with the fact that while providing this cyclists' benefit, the company simultaneously offered all employees a heavily discounted parking pass in the building's basement garage. How do you think most people got to work? (Now--although this example seems to lend itself against my argument by demonstrating that a cheap parking bonus succeeded in encouraging more driving, in that particular environment driving was still designed to be the most convenient choice. The very same garage also had a free bike cage, but for some reason maybe only 3 out of maybe 200 employees ever biked to work).
From what I have observed, TDM is attractive because it offers what look like solutions to reducing driving without actually doing anything that would actually make driving less convenient. While yes, most TDM campaigns at least mention congestion pricing or parking pricing, from what I've seen these rarely make it to the final list of options (and when they do, they are often framed as "sticks" rather than removal of an existing "carrot" for drivers). The result is TDM programs become a complicated, unsustainable web of overlapping initiatives trying to manipulate demand which is formed in commuters' brains, rather than supply, which is right in front of us.
Cities control right-of-way and control the rules which force people to build parking on private property. Both these areas are still predominantly designed and managed to make driving convenient. The apps, special programs, oversight positions, and marketing campaigns feel nice but don't address the fundamental problem about how cities show one set of priorities in the ROW (and parking lots), and another when it comes to how people are actually supposed to travel. This is a reality that US planning, including academia, seems to shy away from addressing directly. As UCLA professor Manville stated clearly in"Travel and the built environment: Time for Change":
The trouble with the transportation and land use literature is that it focuses relentlessly, and sometimes exclusively, on those land uses where transportation does not occur. The 5Ds measure a lot of the landscape, but not the landscape that holds cars.
We could say that TDM focuses relentlessly, and sometimes exclusively, on the things that don't really make a huge difference in how people drive. Like a lot of planning, TDM lets planners avoid dealing with a problem where it originates. People aren't driving a lot because they don't know what time the bus runs or they can't shower at work, they're driving because we spend billions of dollars to make that the easiest choice. Although we don't talk about it, in essence we already have a TDM program in place for which the primary concern is encouraging driving. As Manville, Taylor, and Blumenberg explained elegantly in their 2018 article: "Transit in the 2000s: Where Does It Stand and Where Is It Headed?":
The contemporary United States is designed for and dominated by automobiles. This arrangement has caused undeniable problems, but expecting transit to solve these problems on its own is quixotic. Excess driving is not a result of people failing to take transit; it is a result of people driving. Therefore, the problems of the car will not be solved by transit. They will be solved when public policy confronts the car.
To summarize, TDM should just be called TSM and make clear: we make driving really easy, and if we want to reduce driving, we have to make it less convenient. We can do that most directly by reducing road space, lane widths, vehicle storage areas, or anything else that could be understood to be part of the "supply" of travel. Building more roads encourages more driving. Building more parking encourages more driving. Cities are already statutorily in control of the rules which govern both these spaces, and can build or allow less of both: Why don't we focus on that rather than trying to bend over backwards influencing motorist psychology? I get that it is politically unpalatable, but planners should be providing the best answers, not just the digestible ones.
One final observation: the longer I work in planning, the more I understand how often individual planners' preferences shape the policies they propose (probably me included). But in the office where I work now, who do you think are the biggest proponents of TDM? The ones who drive to work five days a week.