{"id":387,"date":"2018-04-22T00:52:17","date_gmt":"2018-04-22T00:52:17","guid":{"rendered":"http:\/\/ajhassoc.com\/?p=387"},"modified":"2019-04-20T22:33:29","modified_gmt":"2019-04-20T22:33:29","slug":"traffic-delay-from-autonomous-vehicles","status":"publish","type":"post","link":"https:\/\/ajhassoc.com\/index.php\/2018\/04\/22\/traffic-delay-from-autonomous-vehicles\/","title":{"rendered":"Traffic Delay from Autonomous Vehicles"},"content":{"rendered":"<div class=\"entry\">\n<p>A recent post to the TMIP Forum by Suzanne Childress and subsequent replies gave me pause.&nbsp; The question was innocent enough:&nbsp; what VDF (volume-delay function) should be used with a 100% autonomous vehicle traffic stream.&nbsp; Two people answered that this was the wrong question to ask, but I don\u2019t think this question is going away any time soon.<\/p>\n<p>Of course, a 100% autonomous vehicle fleet is science fiction.&nbsp; We have been talking seriously about such a scenario for over 40 years, and we are approaching realization of this scenario at a snail\u2019s pace.&nbsp; The implementation problems are as much institutional as they are technical, maybe more so.<\/p>\n<p>In late 1974, as a freshly minted PhD, I arrived at General Motors Research Laboratories to find, among other things, my new colleagues killing off an unpopular research project on autonomous vehicles.&nbsp; Somebody, and I don\u2019t know who, had publicly stated that it was possible to build computer-controlled freeways that could achieve speeds of up to 150 mph while also achieving flow rates of up to 10,000 passengers cars per hour per lane.&nbsp; GM felt the need to investigate.&nbsp; The staff used the best car-following theories available (GM was a leader in car-following research at the time) and found that these numbers had been ridiculously inflated.&nbsp; GM\u2019s staff required near absolute safety.&nbsp; Even though they could assume drivers were perfect, they could not assume that either the vehicle or the electronics were perfect.&nbsp; Thus, reasonably safe headways between cars were imposed.&nbsp; They picked operating parameters that could be defended in court, hypothetically.<\/p>\n<p>VDFs apply to uninterrupted traffic flow, only, as was pointed out in one response to the question.&nbsp; Intersection delay is much more important in urban settings. &nbsp;Autonomous vehicles would likely require newer intersection delay relationships, too.&nbsp; Reliability is another consideration.<\/p>\n<p>For many years, I gave a homework assignment in my Transportation Engineering class asking students to estimate the very high-speed capacity of a freeway lane by using a vehicle following equation found in our textbook and by adopting conventionally accepted safety standards.&nbsp; (If you are interested in retracing the logic, the textbook is by Papacostas and Prevedouros, our infamous friend Panos.)&nbsp; The capacities were shockingly low.<\/p>\n<p>A much simpler question, I sometimes used as an extra credit problem on exams, is to estimate the service flow at a given speed when everyone adheres to the \u201crule of the road\u201d.&nbsp; There are actually two of these rules depending upon how old you are.&nbsp; When I took driver\u2019s training I was told to leave one car length of space ahead of me for every 10 mph of speed.&nbsp; My children learned to leave 2 seconds of gap, regardless of speed.&nbsp; (The National Safety Council recommends 3 seconds.)&nbsp; In any case, it is possible to derive a VDF from only this information and the length of a car.<\/p>\n<p>So for example, a 2 second gap at 60 mph for a 15-foot long car gives a spacing of 2*88 + 15 = 191 feet.&nbsp; This is a density of 27.6 cars per mile, and using the fundamental equation of traffic flow, 60*27.6 = 1659 pcphpl (passenger cars per hour per lane) flow rate.&nbsp; Not an unexpected number.&nbsp; This is, perhaps, one point on our human-driver VDF.<\/p>\n<p>I have not seen a treatise on the legal issues surrounding autonomous vehicles.&nbsp; Everything is speculative since there is no case law.&nbsp; Automobile manufactures successfully avoid a lot of liability by claiming their vehicles are safe when operated and maintained properly.&nbsp; A crash is almost always the fault of a person, not a machine.&nbsp; But with an autonomous vehicle, a crash is caused exclusively by the machine.&nbsp; To avoid crashes, the system needs to be fail safe under all environmental conditions.&nbsp; A decent set of &nbsp;safety parameters in a vehicle following equation might come from an assumption of the lead vehicle coming to a stonewall stop and the following vehicle braking moderately aggressively after a reasonable reaction\/perception time.&nbsp; So if we were to choose a reaction time of say 1 second and a coefficient of friction during the skid of 0.3 (wet flat road), the required spacing is 1*88 + 88*88\/(2*32.2*0.3) + 15 = 460 feet.&nbsp; This corresponds to a density of just 11.5 cars per lane mile and a flow rate of a paltry 689 passenger cars per hour per lane.<\/p>\n<p>100% means 100%.&nbsp; There can be absolutely no human-controlled vehicles. &nbsp;As pointed out to me by my friend Ed Beimborn, these 460-foot spacings (31 car lengths) would be very attractive to a human driver who could easily weave through them to go much faster than the design speed and those poor occupants who are faithfully allowing their cars to drive themselves.&nbsp; The temptation to defeat the computer controls would be overwhelming.<\/p>\n<p>These analyses suggest to me that autonomous vehicles could never outperform a bunch of native Californians in traffic flow efficiency, given our litigious society.<\/p>\n<p>As always, I am interested in hearing your comments.<\/p>\n<p><em>Alan Horowitz, Whitefish Bay, April 21, 2018<\/em><\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"<p>A recent post to the TMIP Forum by Suzanne Childress and subsequent replies gave me pause.&nbsp; The question was innocent enough:&nbsp; what VDF (volume-delay function) should be used with a 100% autonomous vehicle traffic stream.&nbsp; Two people answered that this was the wrong question to ask, but I don\u2019t think this question is going away &#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":[13,10],"tags":[],"class_list":["post-387","post","type-post","status-publish","format-standard","category-scenario-development","category-travel-modeling","anons"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v27.2 - 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