chore(prototype): passing lanes at 20 m/s to make the normal lane preferred

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/research/vehicle-probe.md

@@ -104,3 +104,4 @@ The human confirmed in game what the log showed: rebuilding the segment under a
   - Vehicles slowed to 9–10 m/s and briefly stopped on segment 27290 (presumably the end of the prototype stretch): a merge from two lanes into one.
   - Visually the lane change is abrupt (human: "the lanes need a smoother transition"): the switch happens at the node, which has almost zero length on a straight, and the jump is about 5–6 m sideways.
   - Next ideas: a lower speed limit on the passing lane (prefer the normal lane unless it's blocked), a smaller offset or transition segments for a smoother change.
+- Third run setup: passing lanes at 20 m/s (normal lanes 22.22 m/s), everything else unchanged. Question: does a lower limit keep vehicles in the normal lane and send them to the passing lane only when the normal lane is blocked?

mod/content/streets/overtake_proto_b.street_template.lua

@@ -1,8 +1,10 @@
 -- PROTOTYPE (ticket "Can a street type with overlaid passing lanes fake overtaking on single-lane roads?"):
 -- throwaway, lives only on branch prototype/vehicle-probe.
 -- Looks like country_new_small (same style, one visible lane per direction), but each direction gets a narrow
--- extra passing lane at the centre line, pushed by `offset` over the oncoming lane. Variant B: passing-lane
--- offsets B -2.5 / F +2.5 (the unit and sign of LaneConfig.offset are unknown; variants A and B use opposite signs).
+-- extra passing lane at the centre line, pushed by `offset` over the oncoming lane. Passing lanes run at 20 m/s
+-- (normal lanes 22.22) to test whether a lower limit keeps vehicles in the normal lane unless it's blocked.
+-- Offsets B -2.5 / F +2.5 put the passing lanes over the oncoming lanes (variant B; the opposite signs, variant A,
+-- were wrong and are removed).
 function data()
 return {
 		availability = { yearFrom = 0, yearTo = 0 },
@@ -21,9 +23,9 @@ return {
 			{ forward = false, height = 0, offset = -1,   speed = 22.22, transportModes = { "PERSON", "CARGO" }, width = 2 },
 			{ forward = false, height = 0, offset = -0.5, speed = 22.22, transportModes = { "CAR", "BUS", "TRUCK" }, width = 5 },
 			-- backward passing lane, meant to sit over the forward driving lane (BUS/TRUCK only: keeps town cars out)
-			{ forward = false, height = 0, offset = -2.5, speed = 22.22, transportModes = { "BUS", "TRUCK" }, width = 0.5 },
+			{ forward = false, height = 0, offset = -2.5, speed = 20, transportModes = { "BUS", "TRUCK" }, width = 0.5 },
 			-- forward passing lane, meant to sit over the backward (oncoming) driving lane (BUS/TRUCK only)
-			{ forward = true,  height = 0, offset = 2.5, speed = 22.22, transportModes = { "BUS", "TRUCK" }, width = 0.5 },
+			{ forward = true,  height = 0, offset = 2.5, speed = 20, transportModes = { "BUS", "TRUCK" }, width = 0.5 },
 			{ forward = true,  height = 0, offset = 0.5,  speed = 22.22, transportModes = { "CAR", "BUS", "TRUCK" }, width = 5 },
 			{ forward = true,  height = 0, offset = 1,    speed = 22.22, transportModes = { "PERSON", "CARGO" }, width = 2 },
 		},