To determine the cold start excess emissions, firstly, the cold start emissions weighted over the shares are calculated for each urban fleet composition used and for each pollutant. For this, the excess emissions per pollutant and vehicle stratum are requested from the HBEFA database.
The distribution of this emission onto links is done in two different ways, which can be switched via attribute Calculate start excess based on routes at the origin zone:
- Polygonal calculation
- Calculation on routes
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Note: In HBEFA, cold start excess emissions are not indicated for all vehicle strata. For strata without an available excess, a cold start excess emission of 0 g/start will be applied. |
Polygonal, geometrical calculation
The idea of the geometrical calculation is that the start of a route is diffuse. In the model, it begins at the origin zone and enters the link network via a connector node. Realistic routes, however, begin at an unspecified nearby location in the network. This is where the cold start excess emission originates, too. And this is used to avoid the calculation on routes as follows.
For each origin zone, firstly, the absolute cold start excess emission is calculated as total over the demand segments over the products from the value of the attraction attribute of the demand segment multiplied by the share of cold start and the emission factor of the respective pollutant for the fleet composition to be applied. This absolute emission per pollutant is distributed length proportionally to all links that are not closed for the PrT which lie within a radius of 1km around the convex hull of the connector node of the zone. Cold start excess emissions which arise from different zones are accumulated.
Calculation on routes
In order to determine the cold start excess emissions on routes, all routes of the demand segments to be calculated are evaluated from the origin to the destination. For each traversed link, a cold start excess share AP,S is calculated as the integral of the decay function over the link length. This share is multiplied with the volume of the route and the share of cold start of the origin zone of the route. Any attribute, whose content does not have to correspond to the total of the volumes of all routes, can be used as volume value when calculating the warm emissions. In order to calculate meaningful cold start excess emissions anyhow, the value is divided by the volume of the demand segment afterwards and multiplied by the value of the volume attribute. That implies that the relation between the route volume and the link volume multiplied by the value of the volume attribute yields the assumed route volume on the link, which, however, does not have to be constant along the route any more. Per link, the value is summed up over all routes. The evaluation of the routes can end as soon as the first four kilometers of the route are traversed, because the decay function is constantly 0 thereafter.
After that, for each link, pollutant, and demand segment, the calculated value is multiplied with the cold start excess emission factor of the fleet composition allocated for urban and projected over AP and AH.
As in the case of the polygonal method, the calculated absolute emission of the zone is then distributed proportionally to this indicator per link onto the links. Please note that this does not yield the exact dynamic route volumes but an acceptable approximation. In order to use the dynamic route volumes in the procedure, the traffic flow model of the used dynamic assignment would have to be reproduced. The volume per analysis time interval calculated from these dynamic route volumes during the assignment is used instead.
Like the other emissions, the cold start excess emissions are aggregated network-wide and issued in the statistics list Emissions (HBEFA).
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Note: If no routes are available for a demand segment and the calculation on routes is demanded at a zone, no cold start excess emissions will be calculated for this zone. Besides the explicit rejection of the routes, this is for example the case if you want to determine emissions of service buses using a separate, artificial demand segment whose volumes result from, for example, the number of service trips. Here, the omission of the cold start excess emissions is in line with the fact that almost all of the trips are warm. The procedure can, however, still be run. |
