Theories of Traffi c Calming and Latent Demand

The concept of highway removal is controversial because of the fear that 1) no other roadway typology can accommodate high enough traffi c numbers; and 2) traffi c will spillover onto local roads and cause backups elsewhere in the system. Critics charge that

“central city traffi c congestion will worsen, and putting more cars and trucks onto surface streets will increase pedestrian fatalities” and that economic gains will be offset by the loss of businesses preferring freeway-served suburban locales.⁴² And yet, in cases where highways have suddenly been closed because of collapse, traffi c repercussions have been surprisingly mild or nonexistent. Transportation offi cials are often at a loss as to how roadway networks adapt and avoid gridlock in cases where a highway has suddenly been closed. In other words, the system may be more adaptable than traffi c advocates think.

This section investigates precedents on which data for traffi c dispersal is available in order to underline the fact that fears of traffi c chaos are often unfounded.

The concept of latent demand and traffi c demand elasticity is at the very center of the concept of right-sizing of urban highways. It has largely been assumed that expansion is the only option; and yet numerous precedents examined here will show that a decrease in capacity is more easily handled than previously believed. It is possible that these cases essentially observed the phenomenon of “triple convergence” in which roadway capacity increases often fail to provide congestion relief, because the newly available capacity is fi lled by traffi c attracted from 1) parallel routes; 2) parallel modes; or 3) shifts in travel time into the peak period.⁴³ A decrease in capacity may therefore exhibit this phenomenon in reverse. It may well be that given the large redundancy of multimodal urban networks, this process is equally valid in absorbing the shock of capacity loss.

While all the reasons are not always clear, two key aspects to capacity decrease are: a balance in tradeoffs – “between mobility and safety objectives on the one hand and urban regeneration and economic development objectives on the other”⁴⁴ and a balanced approach to preparing the city for this change: ‘engineering, education, and enforcement.’⁴⁵

Research within the fi eld of transportation planning and engineering is seeking to understand how supply and demand adjust when capacity is reduced or when a link 42 Cervero, “Freeway Deconstruction and Urban Regeneration in the United States,” 2.

43 Anthony Downs, Stuck in Traffi c: Coping with Peak-hour Traffi c Congestion (Washington, DC: Brookings Institution, 1992).

44 Cervero, “Freeway Deconstruction and Urban Regeneration in the United States,” 2.

45 Cervero, “Freeway Deconstruction and Urban Regeneration in the United States,”7.

disappears from a roadway system. Since the addition of transportation capacity

potentially affects the likelihood of additional trips taken by urban residents, the reverse should occur with traffi c calming and reducing capacity for a parallel reduction in

additional trips taken. Neither relationship – increased capacity and increased demand, or the inverse of decreased capacity and decreased demand – has been unquestionably proven.

Ryuichi Kitamura’s evaluation of existing empirical data on the subject fi nds that models using the standard sequential procedure are:

“capable, in principle, of forecasting diverted, transferred, and shifted traffi c, although actual practice may be less than ideal... Impacts on car ownership, residential and job location choice, and land use need to be better understood and incorporated into the forecasting procedure. More widespread use of panel surveys is encouraged.” ⁴⁶

Thus the lack of understanding on this commonly cited phenomenon points to the additional work to be done in traffi c demand modeling. Although the research has yet to precisely quantify the relationship between traffi c supply and demand, the existing research does show that the elasticity of the network to adapt to change is typically greater than predicted. A study from the United Kingdom researched over 70 case studies of road space allocation in eleven countries and collected opinions from over 200 international transportation experts.⁴⁷ The results suggest that:

“…predictions of traffi c problems are often unnecessarily alarmist, and that, given appropriate local circumstances, signifi cant reductions in overall traffi c levels can occur, with people making a far wider range of behavioural responses than has traditionally been assumed.”⁴⁸

The context for this report is the offi cial shift on road building in the United Kingdom in the 1990s that acknowledged that road expansion was not always a solution to congestion;

on the contrary, building additional capacity could in fact generate traffi c. The acceptance of the concept of induced demand stemmed from the building of the M25 motorway around London, which in spite of increased capacity did not result in traffi c improvement to the expected degree.⁴⁹ In an effort to avoid this problem in the future, numerous improvements to bus and other modes were proposed; yet these were rejected based on fears that

automobile traffi c from those routes would be negatively impacted and/or diverted onto other streets. Since there was little existing research work on whether correlation

46 Ryuichi Kitamura, “The Effects of Added Transportation Capacity On Travel: A Review of Theoretical and Empirical Results” Transportation, (2009) 36:745–762.

47 S. Cairns, S. Atkins and P.Goodwin, “Disappearing traffi c? The story so far,” Municipal Engineer (Proceedings of the Institution of Civil Engineers) 151 (March 2002), 13-22.

48 S. Cairns, et al, 13.

49 S. Cairns, et al, 13.

exists between reduced capacity and reduced demand, the results of such a scheme were unknown. The controversy and the lack of information spawned several studies in 1997 which were re-analyzed, updated, and expanded by Cairns, Atkins, and Goodwin in 2002.

The examples studied included any kind of work that resulted in reduced or reallocated capacity: pedestrianization of downtowns, increased bus lanes, bridge damage, etc. Overall, researchers found these projects exhibited a median reduction in overall traffi c volumes of approximately 11%.

Interviews revealed that while planners and engineers may recognize the possible effects leading to trip reduction, “it is reported that, in practice, many work on the basis that traffi c levels remain fi xed” and that the demand for car trips accommodated in urban planning schemes is far less elastic than it may be in reality.⁵⁰ Planners appear to be more skeptical than necessary since “Controversy… is not always dispersed by technical success.”⁵¹ Even though case studies may prove that highway removal is a technically feasible option, planners still have an uphill battle against the misconceptions about traffi c adaptability and context sensitive design.

Of course, two key facts must be understood in relationship to these fi ndings: First, in the past, traffi c reduction schemes were very conservative and so only the projects with the highest likelihood of success would have been implemented, so the results are skewed for successful traffi c reduction. Secondly, it must also be acknowledged, as the researchers noted, that each project is highly individual in its characteristics, and a wide array of percent reductions is thus to be expected. Since this study covered many European cases, one may approach these results critically from an American perspective, arguing that the traffi c system and land use pattern here is so highly skewed towards the automobile that system users cannot divert to other routes or modes the way they could in other countries. The primary American example used in the study is the closure of a New York City highway in 1973 (presumably the West Side Highway, but it is not specifi ed). This is clearly an environment where alternative modes are readily available in a density not seen many other places in the United States. Thus perhaps a key message here is that traffi c reductions will occur and projects are highly likely to be successful, but only if other travel options are available. This study again points to the need for further research within an American setting and to more concretely defi ne the phenomenon of induced/reduced demand. The uncertainty remaining in the research on this subject may mean that projects proposed with this phenomenon in mind will continue to be the exception rather than the rule.

50 S. Cairns, et al, 14.

51 S. Cairns, et al, 14.