Some tests in OSRM make turn penalties updates with negative value. This introduces incorrect behavior in snapping start points and may lead to quite non-obvious problems like #3647 and #3641.
Practically, negative turn penalties would make turns more preferable instead of going straight with 0 turn penalty. This can be achieved also by using a positive bias value and making all turn weight penalties positive. Duration values are not used as key values in heaps and can be negative.

• clarify domain of turn penalties
• review tests features/car/traffic_turn_penalties.feature and features/testbot/traffic_turn_penalties.feature
• adjust updates

  osrm-backend/src/contractor/contractor.cpp

  Lines 832 to 841 in 005124f

  	if (turn_weight_penalty + new_weight < weight_min_value)
  	{
  	util::Log(logWARNING) << "turn penalty " << turn_weight_penalty << " for turn "
  	<< turn_index.from_id << ", " << turn_index.via_id << ", "
  	<< turn_index.to_id
  	<< " is too negative: clamping turn weight to "
  	<< weight_min_value;
  	turn_weight_penalty = weight_min_value - new_weight;
  	}

• add non-negativity check of weights at

  osrm-backend/src/contractor/contractor.cpp

  Lines 460 to 467 in 005124f

  	auto segment_speed_lookup = CSVFilesParser<Segment, SpeedSource>(
  	1, qi::ulong_long >> ',' >> qi::ulong_long, qi::uint_ >> -(',' >> qi::double_))(
  	config.segment_speed_lookup_paths);
  	auto turn_penalty_lookup = CSVFilesParser<Turn, PenaltySource>(
  	1 + config.segment_speed_lookup_paths.size(),
  	qi::ulong_long >> ',' >> qi::ulong_long >> ',' >> qi::ulong_long,
  	qi::double_ >> -(',' >> qi::double_))(config.turn_penalty_lookup_paths);

• add at

  osrm-backend/src/engine/routing_algorithms/routing_base.cpp

  Line 112 in 005124f

  const EdgeWeight to_weight = weight + edge_weight;

  assertion that to_weight >= 0

/cc @danpat