Transportation Supply & Demand Issues

"Induced" traffic is that growth in vehicle miles travelled that is theorized to be caused -specifically- by an increase in road capacity. Induced traffic is not an increase in vehicle miles travelled caused by population growth, employment growth, change in number of women in the work force, changes in family size, an increase in part-time employment, changes in land use, etc. (Trips that are diverted to the new freeway capacity off of neighborhood/arterial streets are also not "induced traffic", but "diverted traffic".) It is difficult in attempting to measure and predict "induced traffic" to separate the effects of these various factors on the total vehicle miles traveled in a region. Transportation economists generally agree that while there is some "inducement" of traffic with expansion of a corridor's trunk road due to pent-up demand, the amount is dwarfed by the effects of increased regional population and employment on traffic growth.

Opponents of widening Hwy 1 for full-time HOT (High-Occupancy or Toll)-Bus lanes in Santa Cruz argue that such an increase in road capacity will "obviously" and "inevitably" be filled by induced traffic. Many people have based their arguments on their own experience with places such as San Jose and L.A., where they watched congestion increase despite the addition of road capacity. Their anecdotal analyses, however, do not take into account the huge increases in population and employment in these areas. In L.A., for example, population increases have so outstripped road capacity that L.A. has about half the freeway mileage per capita of Portland [Portland Metro's publication "Metro Measured"]. (Santa Cruz County is projected to experience a 45% increase in vehicle miles travelled due to population and employment growth from 1990 to 2015. [S.C.C. MTIS])

Those who look for research to support the theory that roads necessarily get filled with induced traffic cite the Surface Transportation Policy Project's (STPP) Analysis of the Relationship Between Highway Expansion and Congestion in Metropolitan Areas (11/98). The STPP paper is based on a rough interpretation of data from Texas Transportation Institute's (TTI) Mobility Study (of congestion data in the urban U.S.), but TTI's own conclusions do not support the conclusions of STPP. While TTI cautions that road expansion should not be relied upon as the exclusive remedy for addressing congestion, road expansion can successfully be combined with other measures such as HOV lanes, improved transit and road pricing targeted at congestion (such as HOT lanes). In fact, the two communities in the TTI study with the best congestion-reduction performance between 1980 and 1996 -- Phoenix and Houston -- achieved their results through increases in highway capacity. Houston in particular made a major investment in HOV and HOT lanes. (Note that a HOV or HOT lane induces less traffic than a general purpose lane because the lane's capacity is restricted.)

Santa Cruz opponents of widening Hwy 1 for full-time HOT lanes should also note that STPP recommends that metropolitan areas should start investing in more promising strategies to manage congestion such as "... pricing strategies (e.g. roadway tolling) ..." [Analysis of the Relationship Between Highway Expansion and Congestion in Metropolitan Areas, cited above] and "Dedicated bus lanes can help bus riders fly past the worst highway bottlenecks." [Road Work Ahead: Is Construction Worth the Wait?] TTI also states that addressing congestion "will not require only one solution, but a range of strategies [including] bus/carpool lanes" and recommends exploration of variable pricing solutions. [Mobility Study, cited above]

[From a special issue of the journal Transportation devoted to induced travel: