Comment 77396

By continuous ring (anonymous) | Posted May 28, 2012 at 12:19:34 in reply to Comment 77377

A balanced view from:
Evidence on the Effects of Road Capacity Reduction on Traffic Levels
by Phil Goodwin, Carmen Hass-Klau and Sally Cairns

General caveats and problems of interpretation
In every case, there were caveats and problems in obtaining absolutely definitive results. Problems arose because monitoring is usually done for a different purpose. Screenlines for traffic counts are rarely completely reliable and often cover a rather small area, but however large the area there is always the possibility that some changes to even more distant routes are missed. Some counting methods are proportional to the mileage travelled, and these are not always reconciled. Surveys of behaviour usually do not cover a long enough period of time, are not always carried out at the most appropriate intervals, and rarely use techniques which can identify the underlying changes in individual behaviour behind the net changes in aggregate quantities. Available reports, written for specific local purposes, often omit some pieces of information which would have been relevant to this study, and require some interpretation. In addition, in many cases, other transport changes have also been implemented in the same time period, such as opening a new bypass, or improving public transport services.
Four main potential sources of systematic bias were identified. These were:

(1) Day-to-day variability in traffic not allowed for in one-day traffic counts. This is almost certain to result in an overestimate of the range of results from lowest to highest, but would not, by itself, cause bias to expected mean values.

(2) Journey detours may be longer-distance than captured in cordon counts. Logically, such detours are always possible, and would result in some increase in traffic outside the studied area, and hence, an overestimate of the measured reductions in traffic. The likely size of this effect will be influenced by the availability of alternative routes outside the studied area, and the proportion of trips whose origin or destination is sufficiently far away from the affected roads that longer-distance detours are realistically more attractive than any other behavioural response. Selection of counting locations in most studies was decided by local professionals, who considered that they had caught the routes and roads for which traffic effects were likely to be important. For the few examples with surveys where individuals were asked to report on their responses, long-distance detours were not recorded as a very common phenomenon.

(3) Traffic growth occurs due to other factors like increased income and car ownership. If this is not allowed for in before-and-after studies it will lead to an underestimate of the decrease in traffic due to capacity reduction, and this underestimation increases as the period of the study lengthens. The extent to which traffic reduction is underestimated relates to the magnitude of the traffic growth that would be expected as a result of increases in income, car ownership and similar factors, assuming road capacity remained constant. In many circumstances, it is therefore estimated to be in the range of 1 per cent to 4 per cent per year. If road capacity itself has been increased elsewhere on the network, this will similarly tend to result in an increase in traffic masking the effect where capacity has been reduced.

(4) Partial sampling. If a survey-based is confined exclusively to the users of the road before the capacity reduction, it can observe people who reduce their use but will not observe offsetting former non-users who increase their use, resulting in an overestimate of the estimated reduction in travel.

Where the potential sources of bias applied, the analysis did not make adjustments to compensate for these effects, but drew attention to the issues in relation to the particular case-studies. The second and third effects mentioned are those cited most frequently in discussions on interpretation, and can apply to many of the case-studies. They pull in opposite directions, and the crucial question is the net balance between them. For any given relative magnitude of the two effects, the net effect will logically be progressively more influenced by considerations of general traffic growth, the longer the time period of a study. There is therefore a greater possibility of overestimating the traffic reduction effect in the short-term studies, and underestimating it in the long-term studies. This interpretation is reinforced by substantial empirical evidence on aggregate demand elasticities, and is consistent with pervasive evidence on the importance of other behavioural responses in addition to route change.

Permalink | Context

Events Calendar

There are no upcoming events right now.
Why not post one?

Recent Articles

Article Archives

Blog Archives

Site Tools