Modeling net effects of transit operations on vehicle miles traveled, fuel consumption, carbon dioxide, and criteria air pollutant emissions in a mid-size US metro area: findings from Salt Lake City, UT

Abstract

The Utah Transit Authority (UTA) serves Utah's Wasatch Front, a rapidly growing conurbation with a current population of ∼1.8 M people. UTA uses an electronic fare collection (EFC) system that requires riders to tap on as they enter a bus or train and tap off as they exit, as well as an automated passenger counter (APC) system that counts interruptions of infrared beams across vehicle doorways as riders board and alight. We analyzed EFC and APC data for 2016, along with service schedules and routes from General Transit Feed Specification (GTFS) data, to estimate the impact of UTA on the air quality in its service region by accounting for vehicle miles traveled, gasoline gallons equivalent of fuel consumed, and multiple pollutant species emitted. Buses, light rail, and commuter rail were found to collectively offset approximately 1.5% of the onroad emissions from the counties served by UTA due to transit use replacing single passenger vehicle use. These offsets are not homogeneous; ridership drops significantly (∼20%-50% depending on the mode) during the summer months as some of the largest users are educational institutions with noticeable seasonal cycles. Low transit use during the weekend negates some of the air quality benefits as buses and trains travel at lower capacity. Central routes, particularly during peak travel hours provide noticeable congestion, fuel consumption, and pollutant emissions reduction due to trips taken by transit replacing personal vehicles but off-peak, and non-central routes, show lower benefits. Because the light rail is electric, its local air quality benefits are significant due to the electricity being produced primarily outside the airshed. Upgrading the bus fleet to 2010 model, and newer, diesel and compressed natural gas (CNG) buses, as well as modeling an envisioned change to Tier 3 locomotives for the commuter rail system, was found to significantly reduce regional nitrogen oxides (NO x ), fine particulate matter (PM 2.5 ), and sulfur oxides (SO x ) emissions.

The Association of Media and Environmental Variables with Transit Ridership

“…The commute time period was defined as trips boarding between 6:00 and 8:59 a.m. and 3:00 and 6:00 p.m., with non-commute trips taking place outside these hours. This distinction follows previous studied time periods [9] and allows for a differentiation between non-discretionary (i.e., commuting) and discretionary trips. Transit ridership showed consistent drops during state and federal holidays (both actual and observed days), as well as during the weeks of Christmas and New Year's Day, so these data were removed from the analysis.…”

“…To frame these results into context, the FrontRunner is primarily used for longer distance commuting purposes, buses are used relatively evenly for commuting and non-commuting, and TRAX's use falls between buses and FrontRunner. Bus ridership shows a significant seasonal cycle aligned with the periods when both higher education and schools are in session, with FrontRunner showing almost no seasonal cycle, and TRAX ridership following a pattern more akin to buses [9].…”

“…For instance, Gutiérrez et al [14] studied transit ridership at the station level to calculate a measure for "distance-decay", which illustrates how poorly managed time tables, excessive connections, and distance to a local station can diminish ridership. Similarly, Mendoza et al [9] explored the impact of air quality on ridership in urban settings.…”

“…In addition to cash and voucher payments, UTA services use an on/off electronic fare collection system where a user "taps on" with their transport card when boarding and "taps off" when alighting. These data are collected for each passenger trip and provide necessary information, such as temporal and spatial demand, to inform UTA for service improvement decisions and have been used in a previous study to estimate the impact of UTA vehicles on air quality in its service area [9].…”

“…The black border outlines areas within one mile of the Utah Transit Authority (UTA) service area and white lines show county boundaries: (a) UTA service area (yellow), (b) PM 2.5 nonattainment (green) and CO maintenance (dark green), (c) ozone nonattainment (blue), and (d) SO 2 (yellow), PM 10 (red), both SO 2 and PM 10 (orange) nonattainment areas. Reproduced from Mendoza et al[9].…”