Only a per-minute charge targets congestion

Charge on a 5-mile trip in the regional urban peak zone at the same rate levied two ways. 

The two bases are revenue-neutral by construction: the rate card converts each zone's TAG congestion cost at that zone's own measured speed, so they coincide there and the gap either side is the structure alone.
Chart: Centre for British ProgressSource: DfT TAG A5.4.4; DfT CGN0503 zone speeds; CBP AV modelGet the data