There are a lot many people who take prepaid Cabs from the airport.
And some of them do travel in the same direction in 2 different cabs, leading to double fuel consumption,double emissions and also double money.
While booking the airline ticket, the airline asks the customer if he needs a prepaid cab or not and upto which area in the city of arrival.
Through google maps it can be easily tracked who all want to go in the same direction.
Those people could be given the option to car pool, if they wish to, informing them about the cost benefit and the low carbon footprint that it would lead to.
This can reduce the fuel consumption by slight extent especially where there are lot of foreign tourists coming who end up in the same hotels.
The idea could be checked for its viability by further detailed analysis.
1] in the online booking of the air tickets there should be an option can be updated anytime till the arrival of the pax at the destination to indicate the area or hotel in the city where the pax wants to go and also if he/she is interested in carpool. Travel Agents/Cab Operators/Hotels/Airlines can consider coordination in this direction.
2] Cab operators meru,easy cabs etc., in india who operate mainly at the airport for drops should let the other pax know about the single occupant trips in the direction of a particular point in city with the luggage options after obtaining the consent of the pax already booked and wait for 3 - 5 mins so that other interested pax can join. this will help the pax share the fare. 
3] Nominal tax should be imposed on single occupant cabs leaving the airport. SiPaCaTax single pax car/cab tax