Answer:
change speed
Explanation:
The goal would be to put distance between myself (as the Prius driver) and the two cars on the left. The present relative positions make it unsafe for anyone to change lanes.
Perhaps one relatively safe approach is to slow down. This would give the front driver the opportunity to change to my lane, allowing the back driver to safely pass on the left. It may also tempt the back driver to pass on the right (before the front driver changes lanes).
An alternate, possibly less safe, approach is to speed up, getting in front of the front driver sufficiently far to allow a lane change by that driver. In this scenario, the back driver may change to my lane to pass on the right (as I did), risking a collision as the front driver also changes lanes. The reason for choosing this scenario is to place any possible collision behind me, rather than in front of me.
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The choice of approaches may depend on the observed history in this situation. If it developed because I came up on the right of the front vehicle, then my continuing to pass on the right may be preferred. If it developed because the front vehicle passed me, then slowed, perhaps my slowing would be a better choice. If the back driver has been previously observed to pass on the right or pass with small clearances, that needs to be taken into account in the choice of action.
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The legal requirement is to choose the safest option. In general, that will require choosing a slower speed, rather than a faster one.
