2. Methodology and Data
2.1. HERA Methodology: Highway Emissions Assessment
2.2. Case Study: Spanish National Road Network (NRN)
3. Identification of Hot Spot Corridors
3.1. Procedure for Emissions Priority Index
- Total emissions per segment (measured as CO2eq) express the annual emissions produced by the actual traffic flow
- Emissions intensity per segment (measured as CO2eq/veh-km) serves to identify the most polluting segments based on their traffic operation and infrastructure design.
- No priority when TE and EI <0.5
- High priority when TE or EI ≥0.5
- Very high priority when TE and EI ≥0.5
3.2. Identification of Hot Spot Corridors in the Spanish NRN
4. Mediterranean Corridor: A Case Study for Low-Carbon Road Network Strategies
- Operational strategies focus on minimising emissions through speed enforcement and heavy-duty traffic management to promote efficient use of the network (efficient routing and more effective use of road capacity). The aim is to promote optimal fuel use efficiency.
- Infrastructure strategies seek to reduce emissions by improving the road infrastructure through new or improved constructions, for example by correcting the alignment of the road and reducing steep sections in the design phase.
- Scenario I: Light vehicle management scenario is tested by calculating the impact of reducing the speed of light vehicles on toll, free and metropolitan motorways by −10 km/h from the reference scenario on the whole corridor.
- Scenario II: Heavy vehicle management scenario assumes that all stretches with over 15% of heavy vehicles reduce their speed by −10 km/h on toll, free and metropolitan motorways.
- Scenario III: Alignment improvement scenario considers the same network as the reference (same vehicle distribution and average speed) but with 0% slope in the segments.
- Scenario IV: Integration of all strategies assumes that all three scenarios above are integrated into one.
5. Discussion and Conclusions
- The advantages of geo-referenced databases and results. The study uses a GIS to create a platform to combine traffic and infrastructure databases in the HERA input database. While many research efforts have taken advantage of the spatial environment of a GIS to integrate traffic models and emissions models and perform various aspects of transportation-related emissions modelling [21,22,23,24], these have tended to combine emissions inventory methods and GIS to produce maps and files with geo-referenced emissions in a grid format using proxies (i.e., road density, population, and so on). The use of segment traffic flow data to allocate on-road emissions and their representation (vector) reduces the uncertainty associated with downscaling regional or state-level data to such high resolutions [25,26]. The current paper proposes a method to merge geo-referenced databases (traffic and infrastructure data) and obtain emissions maps using an emission model. The total GHG emissions of the NRN for 2012 are shown by a segment in Figure 2. This segment-by-segment representation makes it easier to identify hot spot corridors. An application of GIS to show emissions in a vector format is a worthy contribution improving the decision making process regarding road network management.
- Identification of hot spot corridors. To tackle climate change problems, it is especially important to establish a priority-ordered plan . Strategies can therefore be applied to the most polluting areas in order to obtain the highest savings. The paper proposed an index (EPI in Section 3) which sorts by priority all road segments or corridors with emissions problems. The EPI was obtained for the Spanish NRN and is shown in Figure 3. Seven corridors were recognised as hot spots. These top seven corridors comprise 25% of the network and are responsible for 51% of the total GHG emissions of the NRN. The outcomes show that these inefficient corridors have a high rate of heavy vehicles, high speeds and steep gradients.
- Designing operational and infrastructure strategies using HERA. Reducing road network emissions requires designing and implementing ad-hoc strategies. Emissions assessment methodologies so far have focused on the separate parts of the road systems: exclusively on road traffic operation ; or on the design, construction and maintenance phases; or on infrastructure . However, attention must be paid to the combined effect of the infrastructure and road traffic. For instance, the effect of road alignment on vehicle emissions  or the effect of maintenance works on traffic flow performance . Specifically, HERA is of particular interest for policies and strategies focused on alignment design, speed adjustment, traffic flow management, and fleet composition. It can be used both in the early planning and design stages and when the road is in operation. The study assesses different strategies in the most polluting Mediterranean corridor. The outcomes show that the integration of speed adjustment (both in light and heavy-duty vehicles) and alignment improvements would result in savings of 5.42% compared to the reference scenario. The most effective strategy is speed enforcement for light vehicles, which could decrease emissions intensity by an average of 10 gCO2eq/veh-km, with a speed reduction of 10 km/h. Another important measure would be to improve road alignment in hilly areas.
Conflicts of Interest
References and Note
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|Road Type||Toll Motorway||Free Motorway||Two-Lane Main Road||Metropolitan Motorway||Total|
|Average share of heavy vehicles (%)||12.04%||17.01%||13.19%||8.69%||14.08%|
|Average speed light vehicles (km/h)||106||110||82||95||93|
|Average emissions intensity (gCO2eq/veh-km)||258.5||275.2||231.6||221.7||247.0|
|Total emissions (Kt CO2eq)||3882.7||15,804.1||5554.8||2892.7||28,134.3|
|% total emissions||13.81%||56.17%||19.74%||10.28%|
|1. North Corridor|
|2. Northeast Corridor|
|3. East Corridor|
|4. South Corridor|
|5. West Corridor|
|6. Northwest Corridor|
|7. Mediterranean Corridor|
|Total Hot Spot Corridors|
|Total length (km)||487||1054||736||999||535||938||1788||6537|
|Average share of heavy vehicles (%)||23.76%||23.42%||20.95%||16.04%||12.73%||14.55%||15.81%|
|Average speed of light vehicles (km/h)||97||98||108||106||109||101||100|
|%km gradient > 2%||2.00%||8.59%||0.00%||1.00%||0.00%||11.46%||1.32%|
|%km gradient > 4%||0.00%||0.00%||0.00%||0.00%||0.00%||8.36%||0.22%|
|Total Kt CO2eq||813||2701||1533||2261||927||1308||4719||14,283|
|% total emissions NRN||2.89%||9.60%||5.45%||8.04%||3.30%||4.64%||16.77%||50.69%|
|Average emission intensity (gCO2e/veh-km)||306||315||287||261||259||260||257||275|
|Two alternative routes: toll vs. free motorway or two-lane main road||√||√||√||√||√||√||√|
|Main corridor (AADT > 20,000 veh/day)||√||√||√||√|
|High proportion of heavy vehicles (%HV > 15%)||√||√||√||√||√|
|Mountain route (Segments with gradient > 4%)||√||√|
|Scenario Description||GHG Emissions Results (KtCO2eq/year)||GHG Emissions Savings (%)||Average Emissions Intensity (gCO2eq/veh-km)|
|Two-lane main road||956.923||-||236.256|
|Scenario I: Light vehicle management scenario||4714.237||3.54%||246.212|
|Two-lane main road||956.923||0.00%||236.256|
|Scenario II: Heavy vehicle management scenario||4838.800||0.99%||251.863|
|Two-lane main road||956.923||0.00%||236.256|
|Scenario III: Alignment improvement scenario||4825.777||1.28%||250.180|
|Two-lane main road||942.566||1.50%||232.560|
|Scenario IV: Integration of all strategies||4607.680||5.72%||240.466|
|Two-lane main road||942.567||1.50%||232.600|
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