{"id":609,"date":"2020-08-03T14:24:17","date_gmt":"2020-08-03T14:24:17","guid":{"rendered":"http:\/\/ajhassoc.com\/?p=609"},"modified":"2020-08-07T23:39:10","modified_gmt":"2020-08-07T23:39:10","slug":"six-short-essays-on-highway-path-choice","status":"publish","type":"post","link":"https:\/\/ajhassoc.com\/index.php\/2020\/08\/03\/six-short-essays-on-highway-path-choice\/","title":{"rendered":"Six Short Essays on Highway Path Choice"},"content":{"rendered":"<div class=\"entry\">\n\n\n<p class=\"has-medium-font-size wp-block-paragraph\"><strong>Where are\u00a0All the\u00a0Trucks?\u00a0 Getting Truck Costs Right Is Really Important\u00a0<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Several years ago,&nbsp;I was working with FAF&nbsp;(Freight&nbsp;Analysis Framework)&nbsp;truck&nbsp;assignment results for the upper&nbsp;Midwest.&nbsp; Those early FAF&nbsp;assignments&nbsp;routed trucks according to travel time.&nbsp; I noticed lots of trucks being assigned to the Chicago Skyway.&nbsp; The Chicago Skyway is a short toll road that runs from the south side of Chicago into northern Indiana.&nbsp; Not long afterward I had occasion to drive on the Chicago Skyway en route to Ann Arbor&nbsp;from my home.&nbsp; As I am&nbsp;driving along, I exclaimed to my wife, Shirley, \u201cWhere are all the trucks?!\u201d&nbsp; There was not a truck in&nbsp;sight.&nbsp; It was clear to me then, and also after subsequent research on&nbsp;data from&nbsp;Ohio\u2019s toll roads, that truckers are quite sensitive to tolls&nbsp;in their path choice decisions.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Adhering to an accurate estimate of truck costs is an imperative for doing any worthwhile freight forecasting at the road-segment level.&nbsp; Truck costs are not all that easy to calculate, so I recommend using something like the CFIRE Truck Cost Model, which&nbsp;contains everything needed.&nbsp;<\/p>\n\n\n\n<p class=\"has-medium-font-size wp-block-paragraph\"><strong>There Are&nbsp;Many&nbsp;Failures of the&nbsp;Standard&nbsp;Model of&nbsp;Path&nbsp;Choice&nbsp;for&nbsp;Automobiles&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">There was a slide in the training course for NCHRP Report 187 that pretty much summarized the standard model of&nbsp;path choice&nbsp;for me.&nbsp; I had three&nbsp;pictograms&nbsp;for&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">TIME&nbsp;+&nbsp;COST + DISTANCE&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The pictograms were a clock, a dollar bill and a ruler.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">I always explained to my students that&nbsp;automobile&nbsp;operating costs were correlated with time&nbsp;and distance and could largely be ignored.&nbsp;&nbsp;&#8220;Costs\u201d were tolls,&nbsp;for the most part.&nbsp; Distance correlated with time, which meant distance could be ignored,&nbsp;also.&nbsp; Values of time varied by driver\u2019s income, so if the software is sufficiently capable, driver-class segmentation could be helpful when there are tolls.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Except I was confronted with all kinds of situations&nbsp;where the standard model did not work.&nbsp; Users of my software reported&nbsp;consistent&nbsp;over-assignment of freeway links.&nbsp; People either did not properly evaluate the&nbsp;travel times on freeways or they considered freeway driving to be inconvenient or stressful.&nbsp;&nbsp;Users also reported&nbsp;over-assignments&nbsp;on big bridges, regardless of tolls.&nbsp;&nbsp;I had&nbsp;questionnaire results&nbsp;showing&nbsp;people overreact to travel times&nbsp;in congested traffic and will avoid known bad pavements.&nbsp; People consistently fail to divert, when they should, from&nbsp;highway work zones.&nbsp;&nbsp;One software user told me he even needed to adjust for curvy roads.&nbsp; These are all \u201csystematic\u201d rather than \u201crandom\u201d errors in the standard model.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The early literature on path choice made reference to many \u201crandom\u201d influences.&nbsp;&nbsp;Drivers presumably make mistakes in their travel-time&nbsp;perceptions.&nbsp;&nbsp;Drivers&nbsp;have&nbsp;preference for different scenery.&nbsp; Drivers sometimes need to make unusual stops along the way, taking them off their preferred route.&nbsp; These \u201crandom\u201d influences could be accommodated, I was told, by&nbsp;multipath&nbsp;stochastic traffic assignment techniques, which simply looked for second, third, fourth, etc.&nbsp;options&nbsp;and&nbsp;threw some trips&nbsp;at&nbsp;them, as well.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">There are other&nbsp;random effects, which&nbsp;are&nbsp;under-appreciated&nbsp;and correctable.&nbsp; Models are known to have serious errors in travel time estimates along links, at bottlenecks and at intersection movements.&nbsp; Big errors stem from&nbsp;the&nbsp;common&nbsp;assumption that all trips for a zone originate or terminate at a centroid, a single point within the zone.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">However, a lot of modelers find&nbsp;stochastic methods to be redundant.&nbsp;&nbsp;User-equilibrium&nbsp;(UE)&nbsp;traffic assignment is inherently multipath.&nbsp;&nbsp;In congested portions of&nbsp;a&nbsp;network, trips are spread out across lots of different paths.&nbsp;&nbsp;But&nbsp;a&nbsp;UE assignment does little to help rural areas and other uncongested parts of networks.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Stochastic assignment has some behavioral underpinnings, but it is mostly a kluge.&nbsp; It improves some traffic assignments by wallpapering over&nbsp;a lot of flaws in the standard model, but it does little for&nbsp;those&nbsp;systematic errors I mentioned earlier.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">The standard model&nbsp;of path choice&nbsp;fails in a number of predictable ways.&nbsp; It is&nbsp;far&nbsp;better&nbsp;fixing&nbsp;those problem&nbsp;areas,&nbsp;where possible,&nbsp;than&nbsp;candy coating&nbsp;them&nbsp;with network theory&nbsp;and swallowing&nbsp;hard.&nbsp;<\/p>\n\n\n\n<p class=\"has-medium-font-size wp-block-paragraph\"><strong>Turning&nbsp;Delays are&nbsp;Important to&nbsp;Path&nbsp;Choice&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Drivers are sensitive to delays at intersections when choosing paths.&nbsp;&nbsp;Delays&nbsp;on&nbsp;urban&nbsp;arterial&nbsp;streets are&nbsp;caused, primarily,&nbsp;by intersections.&nbsp;&nbsp;Turning delays are caused, primarily, by opposing and conflicting traffic,&nbsp;by the type&nbsp;of type of traffic controls,&nbsp;and&nbsp;by&nbsp;how those traffic controls are set.&nbsp;&nbsp;The relationships between&nbsp;delays, traffic&nbsp;levels, and traffic controls are&nbsp;pretty well&nbsp;understood.&nbsp;&nbsp;These relationships are&nbsp;still&nbsp;mostly ignored in large regional models in the&nbsp;US.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Clever modelers will often add penalties to left turns at intersections, but those penalties&nbsp;are&nbsp;static and nearly impossible to forecast well into the future&nbsp;without a separate traffic model.&nbsp; While we&nbsp;can hope that these rather sizable errors will be unbiased and wash out during the equilibrium process, it is more likely they will introduce substantial&nbsp;mistakes to the path building&nbsp;calculations.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">In addition, correct traffic&nbsp;control delay relationships have different, usually larger,&nbsp;sensitivities to traffic increases than those VDFs (volume-delay functions)&nbsp;typically found.&nbsp;&nbsp;A direct effect of a&nbsp;relatively insensitive&nbsp;VDF is insufficient diversion from&nbsp;badly&nbsp;congested facilities.&nbsp; Assignments will be wrong, and&nbsp;MOEs created from VDFs will likely grossly underestimate the impacts of traffic growth&nbsp;scenarios.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">If you are keenly interested in having accurate&nbsp;future&nbsp;estimates of an environment impact, such as carbon emissions, you should be&nbsp;very skeptical of what is coming out of most MPOs these days.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Nobody should trust any project-level results from regional models that do not have reasonably correct delay relationships.&nbsp;<\/p>\n\n\n\n<p class=\"has-medium-font-size wp-block-paragraph\"><strong>Overreaction to&nbsp;Congestion&nbsp;Can&nbsp;Affect&nbsp;Drivers\u2019&nbsp;Path&nbsp;Choice&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">In the late 1970s,&nbsp;I conducted a psychophysical scaling experiment in Chicago to determine drivers \u201csubjective value of time\u201d, using the technique of \u201cmagnitude estimation\u201d.&nbsp;&nbsp;In one part of the&nbsp;experiment&nbsp;drivers responded by giving&nbsp;large&nbsp;subjective values of time to driving in congested traffic, similar to those&nbsp;seen&nbsp;for transit out-of-vehicle time.&nbsp; It is generally assumed people do not like being stopped or slowed&nbsp;when traveling.&nbsp; Another&nbsp;explanation is people are unhappy about the uncertainty&nbsp;of an on-time arrival when there&nbsp;are traffic delays.&nbsp; We know&nbsp;travel times vary more as traffic becomes more congested.&nbsp; We also recently learned&nbsp;drivers place a fairly&nbsp;large&nbsp;weight on&nbsp;uncertainty when making travel choices.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">One method, perhaps, of modeling drivers\u2019 overreactions to congestion is to add an\u00a0uncertainty term\u00a0to link impedance.\u00a0\u00a0But adding an uncertainty term makes sense\u00a0only if the travel times are correctly\u00a0estimated\u00a0in the first place.\u00a0\u00a0\u00a0Typical regional models, which eschew traffic dynamics\u00a0or\u00a0use multi-hour peak\u00a0periods\u00a0or\u00a0rely entirely on VDFs,\u00a0will perform badly in estimating congestion delays, thereby undercutting any improvements once might get from including uncertainty.\u00a0<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Simply adding uncertainty to impedance&nbsp;dooms most models from finding true shortest paths between origins and destinations.&nbsp;&nbsp;I am&nbsp;mostly&nbsp;OK with this.&nbsp; It is better to&nbsp;have the impedance roughly correct than&nbsp;to&nbsp;get&nbsp;the path building exactly correct.&nbsp; There are good, efficient algorithms&nbsp;for finding shortest paths with uncertainty in the impedance, but most MPOs don\u2019t have access to them.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">But&nbsp;I am not OK with&nbsp;sloppy path building in&nbsp;technical papers submitted to the scientific&nbsp;literature.&nbsp;<\/p>\n\n\n\n<p class=\"has-medium-font-size wp-block-paragraph\"><strong>Poor&nbsp;Spatial&nbsp;Precision&nbsp;Causes&nbsp;Major&nbsp;Inaccuracies in&nbsp;Path&nbsp;Choice&nbsp;Calculations&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">You may or may not remember the Lexington data set.&nbsp; In the very early days of GPS,&nbsp;that is&nbsp;1996,&nbsp;FHWA sponsored the collection of path trajectories of vehicles from 100 households in Lexington KY. I&nbsp;asked&nbsp;a&nbsp;graduate&nbsp;student, Oliver Jan,&nbsp;to look at variations in path choice within that data set.&nbsp; When origins and destinations were&nbsp;exactly&nbsp;similar, paths were&nbsp;mostly&nbsp;similar.&nbsp; When origins or destinations were in slightly different locations, paths could be vastly different.&nbsp;&nbsp;It seems, path choice&nbsp;is very sensitive to the exact locations of the trip ends.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">&nbsp;I probably did not need the Lexington data set to figure this out, had I been looking for it.&nbsp;&nbsp;All one needs to do is to&nbsp;compare shortest paths from different places in a zone&nbsp;other&nbsp;than the centroid.&nbsp;&nbsp;In a subsequent experiment, I compared a standard centroid-based assignment with a&nbsp;spatially-disaggregated assignment, where every trip\u2019s path began or ended at the nearest intersection.&nbsp; The differences&nbsp;in the assigned link volumes were huge.&nbsp; Clearly,&nbsp;a major source of error in travel models is the&nbsp;use of zones that are too big.&nbsp; Ideally,&nbsp;at least for the traffic assignment step,&nbsp;zones should be no larger than the catchment area of a single intersection node.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Increasing spatial precision is especially helpful&nbsp;in rural areas and small cities where the multipath&nbsp;nature&nbsp;of UE assignment does little to remove lumpy link loadings.&nbsp;<\/p>\n\n\n\n<p class=\"has-medium-font-size wp-block-paragraph\"><strong>OD&nbsp;Table&nbsp;Estimation from&nbsp;Traffic&nbsp;Counts&nbsp;Requires Good Path Choice Calculations&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Computed paths are of little interest to planners, who care much more about computed volumes, turning movements, and delays.&nbsp; My software, in order to reduce run times&nbsp;and storage requirements,&nbsp;does not even save computed paths.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">However,&nbsp;OD table estimation methods need to know how many trips between any origin and&nbsp;any&nbsp;destination use any particular link.&nbsp;&nbsp;To get this information, the software&nbsp;briefly&nbsp;needs&nbsp;exact paths between OD pairs.&nbsp;&nbsp;So good path choice modeling is essential to good OD table estimation.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">Turning movement data could greatly improve OD table estimates, but most cities do not have very much&nbsp;high-quality&nbsp;turning movement data.&nbsp; I have recommended, instead, to employ whatever movement data there is to validate the OD table.&nbsp; If the&nbsp;assignment gets the percentages of lefts and rights somewhat correct, then OD table can\u2019t be too awful.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">People doing OD table estimation without good path choice assumptions will surely be disappointed&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\">I have heard from network theorists that paths in UE assignments are not unique.&nbsp;&nbsp;That is, it is possible to satisfy Wardrop\u2019s first principle equally well with two or more entirely different sets of paths.&nbsp; You could force a model to create a different set of paths by, perhaps, starting the&nbsp;assignment with free travel times instead of LOS C travel times.&nbsp; It is not clear to me whether it makes much difference, given all the other sources of error in the estimation process.&nbsp;<\/p>\n\n\n\n<p class=\"has-medium-font-size wp-block-paragraph\"><strong>Closure&nbsp;<\/strong><\/p>\n\n\n\n<p class=\"wp-block-paragraph\">A good understanding of path choice is essential for&nbsp;a&nbsp;good travel&nbsp;forecast.&nbsp; Unfortunately, there are a lot of travel models failing to understand path&nbsp;choice.&nbsp;<\/p>\n\n\n\n<p class=\"wp-block-paragraph\"><em>Alan Horowitz, Whitefish Bay, August 3, 2020<\/em>&nbsp;<\/p>\n\n\n<\/div>","protected":false},"excerpt":{"rendered":"<p>Where are\u00a0All the\u00a0Trucks?\u00a0 Getting Truck Costs Right Is Really Important\u00a0 Several years ago,&nbsp;I was working with FAF&nbsp;(Freight&nbsp;Analysis Framework)&nbsp;truck&nbsp;assignment results for the upper&nbsp;Midwest.&nbsp; Those early FAF&nbsp;assignments&nbsp;routed trucks according to travel time.&nbsp; I noticed lots of trucks being assigned to the Chicago Skyway.&nbsp; The Chicago Skyway is a short toll road that runs from the south side &#8230;<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"om_disable_all_campaigns":false,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":"","jetpack_publicize_message":"","jetpack_publicize_feature_enabled":true,"jetpack_social_post_already_shared":true,"jetpack_social_options":{"image_generator_settings":{"template":"highway","default_image_id":0,"enabled":false},"version":2}},"categories":[11,5,6,10],"tags":[],"class_list":["post-609","post","type-post","status-publish","format-standard","category-data","category-environmental-impacts","category-project-level-forecasting","category-travel-modeling","anons"],"yoast_head":"<!-- This site is optimized with the Yoast 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