The World Health Organisation (WHO) ranks Delhi as one of the most polluted cities in the world. A city’s air pollution level is determined through annual mean particulate matter (PM), one of the major contributor to poor air quality. Delhi's was recorded at 156 pm by the Central Pollution Control Board in 2014. These levels are seven times higher than the WHO guideline during 2012-14, making Delhi the most polluted city in India. This has a serious impact on the health of its residents. 
Over the last decade, the governments in rule have taken some steps to mitigate vehicular air pollution levels in the city. But recent research studies have shown that the focus of the policies are mainly targeted on the transport sector. They have not taken into account mitigation strategies for other polluting sectors, have ignored the role of gaseous air pollutants such as nitrogen oxides, and have promoted policies for higher use of private vehicles.
Evidence of Scientific Research
A recent research article in Economic & Political Weekly (Goel & Pant, 2016) has analysed the vehicular air pollution mitigation polices in Delhi. The results are startling. Government policies to date have made use of several air pollution combating techniques (relocation of polluting industries outside the city boundary, dumping of old diesel buses, conversion of all public transport vehicles to compressed natural gas, and the implementation of odd-even formula) to improve the ambient air quality index in Delhi. But so far these policies have ignored scientific facts on the major source of poor air quality in Delhi.

Air pollution “source determination” is complex as there are several sources. With regards to vehicular emissions, passenger vehicles like four wheelers and two wheelers, public buses, auto-rickshaws, freight vehicles and heavy-duty vehicles are the main sources of transport sector emissions. But the information about the contribution of the transport sector as a whole to PM levels is not enough to devise effective pollution control policies. 

To understand the real cause of vehicular air pollution and to design effective policies for transport-related pollution reduction, we need more granular information. We need to quantify the percentage contribution of different sources, as well as quantify the relative contribution of different vehicle types to total transport emissions:
This data clearly states that the share of freight vehicle emission is quite high as compared to passenger vehicles.

To go further into the statistics, the data shows that half of private passenger cars emissions to PM are caused by vehicles are older than 10 years.

Thus, the banning of old cars will result in an average reduction of pollution in Delhi by 2.5%. The above policy adopted by the government of Delhi will help to reduce the pollution levels to some extent.

Some studies have shown that vehicular emission contribute to 25% of the total emission levels while 75% is caused by other sectors, mainly industries such as biomass and municipal waste burning. So even outside of transport, the larger problem of other sector emissions has to be solved.
More Policy Bottlenecks
Over the years a series of ineffective policies were implemented in Delhi to mitigate vehicular air pollution. 

In the 1990s, Delhi built more than 50 flyovers. This encouraged residents to buy more private cars, nullifying the impacts of infrastructure enhancement. Also, Delhi’s government dismantled the bus rapid transit (BRT) system, encouraging usage of private vehicles even more. Another ineffective policy has been the restriction on the autorickshaws to 55,000 units in Delhi in 1977 which was later increased to 1,00,000 units in 2011.

The population of Delhi has increased in this time by several folds. Due to lower public accessibility to these cheap means of transport, these policies actually encouraged usage of more private cars. 

Most inter-city diesel trucks and buses tend to use BS–III (the old emission standard in India) compliant fuel because of its universal availability on the highways. Furthermore, the same vehicles ply in Delhi, and emit higher levels of pollutants. This negates the benefits achieved due to improved emission standards for cars in Delhi. 

Also, the odd-even formula has not led to reduction of air pollution to a large extent because private cars only contribute to 5% of emissions. Delhi’s neighbours Noida and Gurgaon did not implement the scheme, nullifying the impacts of pollution reduction. Another negative consequence of odd-even is that it promoted growth in new car purchases by the wealthy and second hand car purchases by others.
Revamping Pollution Policies
On the whole, research proves that the policies to date have not been able to develop a holistic sustainable transport policy framework for Delhi. The government needs to focus more on curbing air pollution by devising mechanisms for other sectors besides transport. It needs to implement measures to reduce private vehicle ownership, and trying capping measures to address the issue. Convincing neighboring cities and towns to implement these standards would be a bonus.