2.3. Emission Modeling—HBEFA

Handbook Emission Factors for Road Transport (HBEFA 4.1) is an emission factor database, which can be used to estimate emissions. It provides emission factors in g veh km⁻¹ for a variety of vehicle categories, drivetrains, traffic situations, cold and warm start, and climatic conditions [37]. The emissions factors considered in this study are total hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides in NO_x equivalents (NO_x), particulate matter (PM₁₀), PM₁₀ caused by non-exhaust emissions such as tire wear (PM₁₀ (non-exhaust)), reported carbon dioxide excluding biofuel share (CO₂ (rep)), total carbon dioxide including biofuel share CO₂ (total), CO₂ equivalents containing CO₂, CH₄ and N₂ (CO₂e), CO₂ equivalents of Well-to-Wheel emissions (CO₂ e WTW) [37]. HBEFA has been chosen given that it is one of the most frequently used models and includes various traffic situations [48]. Models such as COPERT, which define emission factors as a function of average speed, are not appropriate for this study given their limited calibration options. Models which consider variable traffic situations such as TEE could not be used in this study given that most of the required calibration options (e.g., signal settings, queue length) are unknown in this simulation.

HBEFA is not calibrated for the use in American cities. Hence, an emission model specific to Germany has been adopted to ensure that the changes in the model are calibrated correctly. It was not the goal of this research to exactly quantify the amount of emissions but rather to conduct a sensitivity analysis of the emission model to emphasize that even small changes in the calibration of the model can significantly change the results. Thus, it is not important whether the emission model is calibrated specifically for NYC. It is more important that the emission model itself has been tested and validated. The default fleet composition is representative of the vehicles registered in Germany in 2020. The default traffic situation parameters in this study are urban, primary-city non-motorway, maximum speed of 50 km/h, free flow and 0% gradient. The temperature, trip duration, Well-to-Tank (WTT) emissions and parking duration is representative of the German average for light commercial vehicles (LCV) and private motor vehicles (PMV).

Instead of calculating the emissions for locker and home delivery, this study reports the maximum mode share, which is defined as the maximum share of customers who can pick up a parcel by car before home delivery becomes the better option in terms of emission generation. The maximum mode share is calculated as follows:

where

Maximum mode share for private motor vehicles,

Emission factors of light commercial vehicles (warm),

Emission factors of passenger vehicles (warm),

Cold-start emission factors of passenger vehicles,

Share of the parcel pick-up trip of the total trip length (customer),

Distance per parcel by delivery van (home delivery),

Distance per parcel by delivery van (locker delivery), and

Roundtrip distance between customer and locker.

For some of the simulations in this paper, HBEFA is not differentiating between hot- and cold-start emissions and therefore ecc is set to 0 in these cases. For changes in the temperature, parking and trip duration, HBEFA supplies the hot- and cold-start emissions separately. The factor fc is 1 in all cases except in the variation of the total trip length. For the variation of the total trip length only the corresponding share of the cold-start emissions has been considered for the parcel pick-up trip. In other words, if the total trip is twice as long as the parcel pick-up trip, only half of the cold-start emissions are attributed to the parcel pick-up trip.

The figures in this paper allow for a mode share for cars to be larger than 100% to emphasize the sensitivity of the results. Hence, the following equation has been used: