  Based on the reliability budget and percentile travel time (PTT) concept, a
new travel time index named combined mean travel time (CMTT) under stochastic
traffic network was proposed. CMTT here was defined as the convex combination
of the conditional expectations of PTT-below and PTT-excess travel times. The
former was designed as a risk-optimistic travel time index, and the latter was
a risk-pessimistic one. Hence, CMTT was able to describe various routing
risk-attitudes. The central idea of CMTT was comprehensively illustrated and
the difference among the existing travel time indices was analysed. The
Wardropian combined mean traffic equilibrium (CMTE) model was formulated as a
variational inequality and solved via an alternating direction algorithm
nesting extra-gradient projection process. Some mathematical properties of CMTT
and CMTE model were rigorously proved. In the end, a numerical example was
performed to characterize the CMTE network. It is founded that that
risk-pessimism is of more benefit to a modest (or low) congestion and risk
network, however, it changes to be risk-optimism for a high congestion and risk
network.
