Why is the SSA standard lowered for transport operators?

By Dayan, Failrail.sg

Interestingly, the Land Transport Authority (LTA) uses different measures of train service availability for the Mass Rapid Transit (MRT) and Light Rail Transit (LRT) transportation system, even though both fall under the Rapid Transit Systems Act.

Under the licensing agreement between the LTA and the train operators, SMRT Corporation and SBS Transit are required to comply with the Operating Performance Standards (OPS), which stipulates a certain level of train service availability.

LRT follows the System Service Availability (SSA) standard, which measures service availability as a percentage of the total operating time in a month.

MRT, however, is measured by a totally different yardstick altogether. According to SMRT’s FY2014 annual report [PDF], the service availability of MRT rail lines (such as the North-South-East-West Line; or NSEWL) is reported as the rate in which the rail line operated at least 98% of the scheduled train-kilometres weekly. This metric is known as Train Service Delivery.

Why the difference in service availability standards? LRT uses a monthly figure whereas MRT is weekly. LRT measures availability based on operating time, whereas MRT uses train-kilometres.

Furthermore, MRT’s use of train-kilometres as a proxy for service availability can be gamed. For example, to gloss over the impact of long service delays, the train operator can under-report the scheduled train-kilometres in a week, so as to set a lower target. Alternatively, after the occurrence of a major service disruption, the train operator could jack up the number of train-kilometres operated by increasing trip frequency for the rest of the week.

The MRT’s Train Service Delivery standard is also more difficult to grasp than SSA. For example, in FY2014, the North-South and East West Lines (NESWL) achieved 99.82% for Train Service Delivery i.e. out of 52 weeks in a year, the NSEWL only fell below 98% of the scheduled train-kilometres for 0.0936 weeks [(1 - 0.9982) * 52]. Does this number make sense to you? Neither does it make sense to me.


Even if SSA applies only to LRT, it would seem that the SSA standard has actually been lowered from the 99.7% in 2002 to 99.5% today. It is unclear when the standard was revised by LTA.

So what’s the big deal? Isn’t it a very small difference between 99.7% and 99.5% SSA?

Well, not quite. 99.7% SSA is equivalent to no more than a 4-minute delay for 18 continuous hours of operation (a typical revenue day). This works out to be a maximum of 2 hours of delay each month (30 days * 4 minutes = 120 minutes).

The current 99.5% SSA is equivalent to no more than a 6-minute delay for a typical revenue day and therefore, a maximum of 3 hours of delay each month (30 days * 6 minutes = 180 minutes).

So, the difference between a 99.7% and 99.5% SSA is actually a 50% increase in the number of delay hours that the train operator could incur before risking a million-dollar fine from LTA.

For the futility of fines and other LTA oddities, please see my other article: "Why the inconsistency in which LTA chooses to fine some incidents but not others?"