The CO2 effects come mainly from the replacement of petrol / diesel  fleets with low or zero emission fleets. CO2 benefits largely depend on the size of the fleet that is subject to the procurement criteria, and on the stringency of the environmental criteria.

In a European study, CO2 savings were estimated to be in the range of 161kT-2000kT (again, depending on the assumption of the public vehicle fleet that was concerned).

The most longstanding example of electric bus fleets is in China, which has the largest electric bus fleet in the world. A study on China’s ‘New Energy’ buses finds that Category 4 emissions bus with a fuel consumption of 38L/100km emits 1103 grams of CO2 per kilometre, while electric buses and plug-in hybrids emit 943 g CO2/km and 870 g CO2/km amounting to a 15-20% reduction in carbon emissions. In Shenzhen, by 2017 the entire fleet of over 69,000 buses were electric. Using the 2013 emissions as a baseline, by 2017, the buses reduced CO2 emissions by 8.56 million tons.

In Sweden, a joint public procurement arrangement intended to buy 1,250 cars per year for a total of 5 000 vehicles over four years. As of 2016, the procurement process is credited with contributing to over 900 EVs in use. The program estimates that for every 100,000km the electric cars cause 1.3 tonnes CO2 while an equivalent petrol fleet would emit 13.8 tonnes CO2.

Ex-post evaluation studies mostly show a positive effect of the vehicle purchase and car-ownership tax on CO2 reduction. For example, a study for France, Sweden and Germany shows that the tax reduces the average emissions rate by 7.95 g CO2/km in France, 1.56 g CO2/km in Germany, and 0.57 g CO2/km in Sweden. For comparison, in France, the average emissions rates actually declined by 8.70 g CO2/km between 2007 and 2008. In Germany, emissions rates decreased by 1.55 g CO2/km between the first and second half of 2009, and in Sweden, emissions rates decreased by 1.54 g CO2/km between 2005 and 2007. Thus, in all three countries, taxes explain large fractions of the actual changes in average emissions rates.

A study for Norway showed a NOK 1 000 tax increase (about USD 120) is associated with a reduction of 1.13%-1.58% in vehicle registrations, and the responsiveness in car choice to fuel costs is of the same magnitude. The estimated effect of the tax explains the majority (79%) of the reduction in average CO2 intensity in the new car fleet. A point estimate of the elasticity of the CO2 intensity with respect to the CO2 price is minus 0.06, whereas the elasticity with respect to (resulting) car prices is about minus 0.5.

A review of road pricing simulations or schemes found that distance based charges were generally estimated to reduce CO2 emissions by between 5% and 20% (most studies reviewed were in Europe).

A German study finds that employees with company car benefits drive 24 672 km per year, compared to 12 828 km per year for private cars. These differences contribute to CO2 emissions that may be shifted if access to company cars is reduced.

A Canadian study finds that working from home decreases travel time, overall, by 14 minutes, and increases odds of non-motorised travel for at least 30 min, by 77%. Teleworking in Switzerland is estimated to reduce national traffic volumes by 1.9%.

Company specific studies report substantial CO2 savings. AT&T, a company of 67 900 in 2000 saved an estimated 44 000 tonnes of CO2 due to reduced commuting. According to a 2015 Dell study teleworking employees save 1.15 metric tonnes of CO2 per year (after accounting for negative rebound emission effects, increased energy use at home, etc.).

Carbon taxes are aimed at reducing the fuel consumption which in turn results in decreased CO2 emissions from motor vehicles uses. They also encourage people to shift to cleaner fuel and more fuel-efficient technologies which also decrease CO2 emissions.

For example: In Sweden, in the year 2005, reductions in CO2 emissions associated with the carbon tax gave a reduction of 6.2%, or -0.17 metric tons of CO2 per capita from 1990 (compared to a no carbon tax scenario). The tax level in 1991 was set at $30 per ton of CO2 and then successively raised to the current level of $132 by 2015. Additionally, in 1990, a VAT of 25% was also added to the price of gasoline and diesel.

Another study found that a 10 cent per gallon gasoline tax increase would decrease U.S. carbon emissions from the transportation sector by about 1.5%.

The CO2 emission effects depend on the measure implementation. Providing continuous access to the HOV / low-emission lane seems to improve the CO2 reduction efficiency, while limited access reduces it. A study has evaluated a CO2 emission reduction of about 37% for medium-sized European cities based on observations in Aveiro, Portugal in 2012, while others considering car fleet evolution effects conclude that the effect of the measure on CO2 emissions is not significant.

The average load factors tested and potential small percentage of empty kilometres of these vehicles (~10%) can lead, with average occupancy levels around 7-10 passengers and vehicles up to 16 seated passengers, to CO2 emissions per passenger-km of 4-7 smaller than the average private vehicle. With a mode share of around 20%, 50% of them migrated from car, can lead to CO2 reductions between 10-20%. Moreover, if these vehicles replace conventional large buses with low occupancy levels either to feed public transport or for direct service, these reductions can also produce an additional 5-10% CO2 saving.                             

CO2 benefits are different for different type of services:

Ride hailing can lead to an increase of vkms due to the empty kilometres performed by drivers. This increment depends on the size of the market, fewer vehicles will normally mean additional kilometres are required to pick-up the clients. The additional km can reach 50% in some situations but in a stabilised market these values are closer to 25%.

Ride sharing solutions can ensure a more efficient use of vehicles with higher load factors. Increases in load factor of 50% can produce vkm savings of 30% and similar CO2 emissions per passenger-km. Yet, some literature already points out some potential rebound effects derived from household relocation and travel costs reductions that can absorb between 50-70% of these saving with additional travel or longer trips.

Estimating the impact of bus lanes (with or without TSP) on CO2 emissions is difficult, especially when taking into account the effect they may have on traffic overall. It is often assumed that express lanes lead to a reduction of CO2 emissions, but results depend on road network and mobility patterns (for instance through the possible increase in congestion for other vehicles). Mitigation potential can also vary across space. Although CO2 reductions from traffic management measures could be substantial on one particular road, their wider impacts across the transport network as a whole could still be marginal, or even negative.       

Electrification of rail removes all of the CO2 emissions or other pollutants produced by rail, at least in a tank-to-wheel component. Diesel, the alternative power source for trains emits 2.68 kgs of CO2 when burned. In terms of CO2 emissions per passenger km, the value ranges due to engine efficiency and load factor. Common estimates place CO2 emissions per pkm in the range of 6-30 grams.

The use of electricity will also increase any emissions caused by the production of electricity and will depend on the energy mix of each country.

It is also worth mentioning however, that, as with any infrastructure development, electrification of rail will cause CO2 emissions in the implementation phase.

The energy recovered from braking does not reduce or affect tank-to-wheel CO2 emissions. It is used to power on-board train functions (such as lighting).