The Øresund Bridge was designed by the Danish engineering firm COWI. The justification for the additional expenditure and complexity related to digging a tunnel for part of the way, rather than raising that section of the bridge, was to avoid interfering with air traffic from the nearby Copenhagen Airport, to provide a clear channel for ships in good weather or bad, and to prevent ice floes from blocking the strait. The Øresund Bridge crosses the border between Denmark and Sweden, but in accordance with the Schengen Agreement and the Nordic Passport Union, there are usually no passport inspections. There are random customs checks at the entrance toll booths entering Sweden, but not when entering Denmark. Since January 2016, checks have become significantly more stringent due to the European migrant crisis.