Ideas for a satellite-based electronic road pricing system are being evaluated by the Land Transport Authority (LTA).
Four consortiums submitted proposals after testing various technological solutions between June 2011 and December last year.
The next-generation ERP, which Transport Minister Lui Tuck Yew said is likely to be feasible but still several years away, could charge motorists based on how far they travel on congested roads.
A spokesman for the authority said ideas submitted by the consortiums could form the base for developing the system. He added: "We will still need some time to review and evaluate the performance of the proposals... before we can make a final decision on the way forward."
Each proposal will be judged according to factors such as cost, design and accuracy. The four consortiums are: Kapsch TrafficCom; MHI Engine System Asia & NCS; ST Electronics (Info-Comm Systems) & IBM Singapore; and Watchdata Technologies & Beijing Watchdata System.
They received $1 million each in funding to develop Global Navigation Satellite System-based solutions that could be used for road pricing in Singapore's urban environment.
The authority's spokesman said the trial included live demonstration tests where the consortiums tried out their proposed solutions in a real traffic environment.
More than 100 vehicles were used, with each staying on the road for an average of about 40 days. Preliminary assessments showed a satellite-based system can work in the Singapore environment, said the spokesman. He added that the consortiums also demonstrated "various value-added services that are relevant and useful to motorists and companies".
Last week, Mr Lui said drivers may be able to use a new monitor in their vehicles to pay for roadside parking or for using off-peak cars. Other possibilities include tracing stolen vehicles and getting insurance companies to offer lower premiums for motorists clocking lower mileage, he added.
The consortiums also proposed and tested techniques to mitigate the "urban canyon" effect and track vehicles more accurately in areas with tall buildings that interfere with satellite signals. These included placing beacons at roadside kerbs and detectors on street lights.
Associate Professor Lee Der Horng from the National University of Singapore said the accuracy of signals in the city area is one main concern that arose from the trial. He added: "Since they are talking about pricing, they have to know each vehicle's precise location."
The LTA spokesman said "suitable positioning augmentation techniques" will be implemented to make sure the planned charging scheme will work in areas with tall buildings.
