## 5 Summary  
On the basis of the conducted research, the following conclusions were drawn:  
1. The availability coefficient of buses of seven makes and models was determined for the 12 year period of their operation. The average availability value in the period under consideration was in the \(AT\approx\) (81.03-92.7)% range, depending on the make and model of the bus. The highest average availability was achieved by C and F make vehicles--\(M(AT_{c})\) vehicles = 92.7% and \(M(AT_{\rm F})\) = 91.5%. Both makes came from a high-profile manufacturer.
2. A ranking of the downtime-type categories in the vehicle servicing system at the enterprise was created according to the criterion of downtime repeatability.
3. The relationship between the category of downtime and the bus unavailability coefficient was determined as the basis for rationalization of bus fleet maintenance.
4. It was shown that the share of categorized unavailability was strongly, negatively dependent on the category of downtime ([PERSON]'s rank coefficient R\({}_{\rm s}\approx\) (\(-\)0.6 \(\div\) 1.0)) and mostly slightly positively dependent on the age of the buses (R\({}_{\rm s}\approx\) (0.2 \(\div\) 0.4)).
5. The largest share of the state of unavailability had downtime due to repetitive causes, and therefore for predictable reasons--NB (current repair). Unpredictable, difficult-to-manage downtimes were dictated by random WS--\"engine replacement\" and INN--\"other\" causes.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Vehicle**} & \multicolumn{8}{c}{**Downtime Categories**} \\ \cline{2-8}  & \(\sum\)**–All** & **NB—** & **ON—** & **BCZ—No** & **NC—** & **NW—** & **WS—Engine** & **INN—Other** \\
**Make** & **Factors** & **Current** & **Pending** & **Spare Parts** & **Accident** & **Repair** & **WC—** & **WSL—**Engine** \\  & **Repair** & **Repair** & **Repair** & **Repair** & **Repair** & **Repair** & **Repair** & **Repair** \\ \hline A & 0.51 & 0.41 & \(-\)0.65 & 0.09 & 0.01 & 0.46 & \(-\)0.48 \\ B & 0.22 & 0.49 & \(-\)0.40 & 0.36 & \(-\)0.21 & & \(-\)0.59 \\ C & 0.11 & 0.30 & \(-\)0.44 & 0.03 & \(-\)0.1 & & \(-\)0.13 \\ D & \(-\)0.07 & 0.05 & 0.17 & \(-\)0.4 & \(-\)0.11 & 0.36 & \(-\)0.26 \\ E & \(-\)0.12 & \(-\)0.02 & \(-\)0.22 & 0.09 & \(-\)0.13 & 0.18 & 0.01 \\ F & \(-\)0.19 & 0.21 & 0.11 & 0.07 & 0.22 & & 0.13 \\ G & \(-\)0.04 & 0.26 & \(-\)0.25 & 0.13 & \(-\)0.31 & & \(-\)0.40 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Spearman’s rank correlation coefficient between categorized unavailability and the age of the bus.  
The availability of vehicles significantly affects the economy of transport services. With the increasing availability of vehicles, the unit's fixed costs decrease. The growing availability of vehicles increases the competitiveness of the carrier.  
Conceptualization, J.R. and A.N.; methodology, J.R. and A.N.; software, P.H.; validation, J.R., A.N. and E.D.; formal analysis, A.N. and E.D.; investigation, J.R. and P.H.; resources, J.R. and P.H.; data curation, J.R. and P.H.; writing--original draft preparation, J.R.; writing--review and editing, J.R. and A.N.; visualization, J.R.; supervision, A.N.; project administration, E.D.; funding acquisition, J.R. All authors have read and agreed to the published version of the manuscript.  
The project/research was financed in the framework of the project Lublin University of Technology-Regional Excellence Initiative, funded by the Polish Ministry of Science and Higher Education (contract no. 030/RID/2018/19).  
Not applicable.  
The authors declare no conflict of interest.